Add Python identifiers for the env command

[ghost]

Hello! 👋

At the moment, Python scripts without an extension get ignored by Tokei, so if you have scripts taking the role of executables in the same fashion as shell scripts, Tokei won't count those files properly.

So, this PR will make no-extension files with this line of code work as expected for end-users:

#!/usr/bin/env python3

This PR adds identifiers for python, python2 and python3.

[ghost]

I read some previous issues (#208 and #186) relating to this.

What do you think about having more version identifiers? I only included the 3 most common ones.

Do you think that more version identifiers would be necessary for completeness sake? If you don't mind the big list, I can add all of the minor versions up to 3.6 to the list before the potential merge.

[XAMPPRocky]

Thank you for this PR! Can you rebase this change onto the current master so it can pass the travis tests?

[ghost]

Done. Waiting for Travis.

[ghost]

Any idea on why the NetBSD build failed?

[XAMPPRocky]

@seiks I’ve filed an issue on trust about it it can be ignored.

[ghost]

In that case I feel that this PR can be merged. All the other builds succeeded.

[XAMPPRocky]

Thank you for this PR!