Add support for saving the current configuration settings as a named
configuration and reloading such a configuration later.

All of the configurations are stored in a settings file, by default:
~/.config/pprof.json

A struct named config replaces the old map[string]string which used
to store the set of configurable options. Instead, each option is
now a field in the config struct.

The UI now has a new 'Config' menu with

Save as: prompts for a name and saves current config.
Default: applies default config.
$X: for every config named X: applies settings from X.

The currently selected config is highlighted.

Every named config has a delete button to control deletion.

Some semi-related changes:
1. Both filefunctions and functions in the granularity group had an
   initial value of true, which is incorrect since these are mutually
   incompatible choices. Set filefunctions' initial value to false.
2. Renamed the group for sorting from "cumulative" to "sort".
3. Store testing.T in TestUI instead of leaving the field nil.

Fix comments according to best practices by effective go

Signed-off-by: CodeLingo Bot <bot@codelingo.io>

* record diff base profile label key/value constant (#384)

* address comment

The code was updated in #335 but the doc wasn't updated. Also the doc
didn't include $path which is searched as well, see locateBinaries in
internal/driver/fetch.go.

Add a flag to append a comment to a profile.

* doc: make it obvious that host and port are both optional

* fix minor issue

* address comments

Docs are stale, -http flag now supports host:port.

* Allow -http to accept host:port

* Add serveWebInterface tests.

* Make sure serveWebInterface is serving on the URL.

* Avoid nil pointer dereference

* Simplify the -http flag test.

* Use the right capitalization for hostport.

* Fix the double escaping in regexps.

* Address review comments.

* Added an interactive web interface triggered by passing -http=port
on the command line.  The interface is available by visiting
localhost:port in a browser.

Requirements:
* Graphviz must be installed.
* Browser must support Javascript.
* Tested in recent stable versions of chrome and firefox.

Features:
* The entry point is a dot graph display (equivalent to "web" output).
* Nodes in the graph can be selected by clicking.
* A regular expression can also be typed in for selection.
* The current selection (either list of nodes or a regexp)
  can be focused, ignored, or hidden.
* Source code or disassembly of the current selection can be displayed.

* Remove unused function.

* Skip graph generation test if graphviz is not installed.

* Added -http port and the various modes of using pprof to the
usage message.

* Web interface now supports "show" option.

* Web interface automatically opens the browser pointed at
the page corresponding to command line arguments.

* Some tweaks for firefox.

* Handle review comments (better usage message, more testing).

* Handled review comments:

1. Capture and display errors like "Focus expression matched no samples".
2. Re-ordered buttons to match other interfaces.
3. Use UI.PrintErr to print error messages.

* Handle javascript code review comments (a bunch of cleanups).
Also added pprof binary to .gitignore.

Not all OSes have $HOME variable.
Use appropriate variables per OS.

Fixes https://github.com/golang/go/issues/18864

Also update the build instructions to use go get