Add command-line editing support for interactive pprof

This implements basic command-line editing, using
github.com/chzyer/readline. This will not affect the golang
distribution as it uses its own pprof driver, with its own
driver.

Only tested on Linux.
TODO: Implement auto-completion and persistent history.

* Modified variable type to have []string value

* added repeatableStringKind

* modify repeatableStringKind to actually handle multiple values

* Modify applyFocus to work with multiple values for tagfocus/tagignore

* Modify applyFocus to work with multiple values for tagfocus/tagignore

* Modified CompileTagFilter to handle tag filters of the form -tagfocus=key1=val1

* Modified StringList

* add testing for compileTagFilter

* fixed typo

* fixed code style error

* Revert "fixed code style error"

This reverts commit 0b1826c163.

* Revert "fixed typo"

This reverts commit 2593652bd0.

* Revert "add testing for compileTagFilter"

This reverts commit 152d7b0363.

* Revert "Modified StringList"

This reverts commit b57d35a762.

* Revert "Modified CompileTagFilter to handle tag filters of the form -tagfocus=key1=val1"

This reverts commit ebe1e344bf.

* Revert "Modify applyFocus to work with multiple values for tagfocus/tagignore"

This reverts commit 3b62091b67.

* Revert "Modify applyFocus to work with multiple values for tagfocus/tagignore"

This reverts commit 7172159cd3.

* Revert "modify repeatableStringKind to actually handle multiple values"

This reverts commit abb1e3ec4b.

* Revert "added repeatableStringKind"

This reverts commit 8b8e4e76fc.

* Revert "Modified variable type to have []string value"

This reverts commit 5f213dc4d1.

* added support for filtering by specific tags

* Fixed formatting problems

* Updated variable name for clarity

* Added test cases for tag filtering

* Update tagfocus and tagignore description

* Fixed formatting error

* Updated docs to explain new tag filtering

* Updated docs to clarify tag filtering

* Updated docs to clarify tag filtering

* Updated docs and help comments for tag filtering

* Updated docs for tag filtering

* Update pprof docs for tag filtering

* Update tagfocus and tagignore help descriptions

* Update tagfocus and tagignore descriptions in docs

* Update tagfocus and tagignore descriptions in docs and help

* Updated tagfocus and tagignore descriptions in docs; added TODO for regexp matching

* Updated tagfocus and tagignore descriptions in docs

* Modified docs fo tagfocus for clarity