Plugin for iTerm2 (#3790)

* Plugin for iTerm2 on OSX

iTerm2 is a popular terminal emulator for OSX (https://www.iterm2.com).

The plugin currently implements one function, iterm2_profile, which
allows easily changing the currently selected user settings profile,
without creating a new tab or window, just by calling the function.

For example, this is handy for switching from a dark to a light colored
profile without having to re-open anything.

In addition, it also works within tmux running inside iTerm2.

* iTerm2 Plugin: Run any iTerm2 command

Refactored the plugin so that it can run arbitrary iTerm2 commands.
Should work with any of the supported commands, see
https://iterm2.com/documentation-escape-codes.html

* iTerm2 Plugin: Add functions to change tab color