Show watcher help menu when "h" key is pressed

[mattbrictson]

Pressing "h" while the watch command is running will now display this help message and menu:

`mt --watch` is watching file system activity and will automatically run
test files when they are added or modified. If you modify a source file,
mt will find and run the corresponding tests.

You can also trigger test runs with the following interactive commands.

> Press Enter to run all tests.
> Press "a" to run all tests, including slow tests.
> Press "d" to run tests for files diffed or added since the last git commit.
> Press "h" to show this help menu.
> Press "q" to quit.