Helper to pull in extra github repos with more helpers

[phodge]

Requires #16.

A helper that looks like InstallHelpers('phodge/homely-vim-helpers'). This tells homely to immediately download that repo of helpers and update to the latest version.

Some conventions for a Helper Library:

• Repo layout - need to have a standard place to find the python code
• We should be able to use this on pypi packages also
• Must have a convention for tag names / branch names, or else allow user to choose

Other things:

• Helper API - this needs to be
  ** tidied up and finalised
  ** versioned to allow future expansion
  ** Documented
  ** Improved with tools that assist contributors in developing and debugging their helpers
• Cleanup - we should remove helper libraries that aren't being requested any more (unless we can see that we imported a file from it during execution)
• Can we detect tracebacks coming from helper libraries and roll back to a previous version automatically?
• Build a section of the site that documents or links to 3rd party helpers
• Document how contributors can submit a PR to have their helper included on the main site