Support ignoring submodules

[luxaritas]

Typically, git submodules are external dependencies and aren't desirable to be considered when sizing a repository. cloc --vcs git for example excludes submodule paths by default.

[boyter]

Any reason you would not consider a .ignore file for this sort of situation?

[luxaritas]

Convention, mostly - scc already automatically picks up on gitignore, so I would expect it to behave "as expected" by default (though I recognize there could be disagreement on expected behavior here). cloc doing this also seems to lend credence to it being expected behavior.

Also IMO worth considering that this information is already available in a well-defined manner