Allow filename matching by regexp, or extensions not beginning with '.'

## Describe the enhancement

Filenames ending with `-file_contexts` (e.g. `apex.test-file_contexts` or `com.android.wifi-file_contexts`) should be classified as SELinux Kernel Policy Language. As far as I can see there is currently no way to express this in `languages.yml`, as it requires that extensions begin with '.'. Either relaxing this requirement or allowing regular expressions in the `filenames` field would allow them to be classified properly.