Adds detection of color and pager hg extentions and modifies diff command

If the user's ~/.hgrc or the projects .hg/hgrc enables the use of the color or pager extension, this plugin will not function unless the following option is set.

    let g:signify_diffoptions = {'hg': '--color never --pager never' }

This change enables the plugin to detect if either extension is installed and if so, incorporate the appropriate disabling flag.

NOTE: This should detect cases where the plugin is enabled on a specific project.
NOTE: This should work on windows.
NOTE: Since the output of help is parsed, this change could break if the output of the help command changes significantly.