Clarify what lineman plugins should do

[Foxandxss]

Creating a new plugin with a custom task under /tasks is not working (see Foxandxss/lineman-protractor#2).

There is something wrong on the core, we should investigate that.

[davemo]

I think there's a higher level discussion we should have about what role plugins should play and whether supporting the /tasks folder is part of that. Right now plugins are pretty much simple grunt config mutators but with the creation of the lineman-vendor-split plugin and #344 perhaps we should revisit what plugins should do.

My $0.02 is that they should be as unintelligent as possible :)