Add a way to specify external source for dev-mode

[vmrob]

When a user would like to modify a needy-managed library, they may wish to develop the library from an external location. I would suggest utilizing dev-mode for this:

needy dev-mode mylib --source=../mylib

When this is done, the sources properties for that library should be ignored and instead the library should be treated as if it had a directory: '../mylib' library property.

[vmrob]

Are there any special considerations to allow this to work? I'm hoping to quickly add this feature in order to make working with nested needy-managed projects less tedious.