Fixes loading non-js files from local bosco

When a file which is not a .js file is found within the local
bosco `commands` folder bosco crashes when mistyping a command.

This happens a lot when developing local bosco commands on a mac
as .DS_Store are created willy nilly.
Merge pull request #44 from tomgco/patch/commands

Fixes loading non-js files from local bosco