Improve path discovery of scripts installed through pip

[sergiocorreia]

When e.g. pantable gets installed, pantable.exe is placed within $PATH, but panflute can't find it

However, panflute could just do import pantable; doc = pantable.main(doc).

On Python 3.4+, we can check this with:

import importlib
spam_spec = importlib.util.find_spec("spam")
found = spam_spec is not None

(or we could just try to import it..)

[ickc]

Why not give it a try? ;)

[sergiocorreia]

try is probably simpler and more pythonic, agreed.

Would like to set up some hours to update misc. stuff in the next week(s) so let me know if there are other things we should do.