Standard mechanism for building/incorporating shpec plugins

[rylnd]

There have been cases where a really cool function has been written that doesn't belong in shpec proper but would be great as an optional library.

What I'd like is a standard way to build and incorporate these types of functions into your test suite.

[rylnd]

Here's what I'm thinking: shpec plugins are just repos with a certain file structure. For now, they simply define functions that users can call from their tests. Similar to custom matchers, if we define a standard place to look for them we can just source the relevant files when shpec is run.

Adding plugins is then just a matter of cloning said repos to a particular place. We could eventually build tools that make that process nicer for the user (I'm thinking of things like vundle for vim).