Create plugin architecture and manager #25

Ideas:

1) Provide an interface to implement which accepts an FsEye WatchModel instance 
and returns a Control or Form instance which can be shown by FsEye. Perhaps the 
plugin itself will be held by FsEye as a singleton and have configuration 
properties like context menu text (I am envisioning plugins being invoked via 
right-click context menu on a watch node) and modal vs. non-modal display.

2) Have a plugin manager where you can enable or disable plugins. Plugins can 
be enabled by default and picked up by FsEye by putting them in a "plugins" 
directory relative to the FsEye dll (make sure to recursively traverse 
sub-directories in case plugin developers want to deploy multiple files 
together).