Specify lexer for extra_filetypes

I'm using the `.inc` extension for include files (and the `extra_filetypes` option to include these in the documentation). However, I guess Pygments interprets this as some other kind of file and is highlighting it weird. It would be nice to somehow specify the lexer to use for these files, maybe as another option in the declaration, so:
```
extra_filetypes: inc ! ABC
```
where `ABC` is whatever info Pygments needs to treat it as Fortran.