Initial, barebones tangelo-plugin script.
Created subcommand infrastructure
Don't be clever with enable/disable
Adding infrastructure for "batch" subcommand
Adding abstractions for running pip and git
Added "install" subcommand implementation
Added "enable" subcommand implementation
Knocked a lot of bugs out

Put the Config class back where it was - it should not have been moved in the
first place.  I accidentally took the Config class to be a class representing
*plugin* config.
Converted plugin.conf to list-style
Implemented "list-style" handling of plugin config file
Converted plugin docs to discuss yaml format
Merge branch 'plugin-conf-list' into tangelo-plugin

Closes #425
Treating empty plugin file as a zero-length list of plugins, rather than a null value
Merge branch 'tangelo-plugin' into plugin-system