Support way to filter data files

[analog-cbarber]

As things stand currently, flit will include any file in your source package other than your .pyc files.

This often works, but there can sometimes be other files there which you may not want to include in your distribution.

Support for a package_data option would be sufficient.

This is not quite the same as issue #358, since that appears to be asking for a way to include external data files.

[takluyver]

As things stand, excluding files from your sdist should mostly work, since both flit build and python -m build default to building an sdist first, then unpacking that and using the resulting files to build a wheel. But with either tool, asking them to build only a wheel will bypass that. We should probably fix that discrepancy.