Add the ability to specify an editor in _config.yml

This commit adds an 'editor' subkey to the configuration options for poole.
Using this subkey to specify an editor will cause that editor to open when
'poole post' or 'poole draft' is invoked.
rework to use editor environment variable
Merge branch 'qrohlf-external-edit' (closes #5)
Bump to version 0.6, external editor.