For certain formats (eog, evince, gv, web, weblist), the visualizer
would be invoked regardless of whether the user specified a specific
output file to write to. In order to ensure that output is properly
written, the invokeVisualizer function has a check for whether the
output is os.Stdout and alters behavior based on that. This is the
wrong abstraction layer to do that work.

At the core of the problem is that PostProcess is used to both
do data post-processing that is inherent to the format, and also
to invoke some visualization for the data (to Stdout or browser).
We split these two steps apart and make it obvious which is which
by adding a visualizer field to Command.

This seperation of concerns allows us to simplify the code.

By default hide entries in the comment field that starts with #, and
add a "comments" command to show all comments.

Only create $HOME/pprof if a profile is stored remotely
and must be stored. Also, create non-profile temps on
os.TempDir() instead of $HOME/pprof or /tmp.

Rather than print options on each prompt, add a "options" command
that prints the current options in a user friendly format. Also,
make sure that those options can be parsed back as printed.

This enables pprof extensions to update the default for a variable
before the command line is processed.

Currently pprof will look for source files only on the current directory
and its parents. This makes it hard to examine sources on jobs where
there are multiple source trees (eg from different libraries).

Add a variable to provide a search path for source files. It will default
to the cwd, so there will be no change in behavior by default.

This reverts commit 9c191ebbce.
The trimproto option is no longer needed, removing it to avoid code growth.

Add new trimproto option that generates a new profile while removing
symbol information for functions below nodefraction. This reduces the
profile sizes significantly.

Add field for profile sources to indicate what is the primary
sample value, which visualizers should display by default.
Previously there was a convention for pprof to use the last
sample_index by default, this provides more flexibility.

Also update the build instructions to use go get