Support Cymraeg language (Welsh)

[fin-w]

This adds support for Welsh, based on trigrams generated from the 11+ million Welsh words contained in the Corpws Cenedlaethol Cymraeg Cyfoes https://corcencc.org/, which is licensed under Creative Commons Attribution Non Commercial Share Alike 4.0 International.

Citation:
Knight, D., Morris, S., Fitzpatrick, T., Rayson, P., Spasić, I., Thomas, E-M., Lovell, A., Morris, J., Evas, J., Stonelake, M., Arman, L., Davies, J., Ezeani, I., Neale, S., Needs, J., Piao, S., Rees, M., Watkins, G., Williams, L., Muralidaran, V., Tovey-Walsh, B., Anthony, L., Cobb, T., Deuchar, M., Donnelly, K., McCarthy, M. and Scannell, K. (2020). CorCenCC: Corpws Cenedlaethol Cymraeg Cyfoes – the National Corpus of Contemporary Welsh. Cardiff University, http://doi.org/10.17035/d.2020.0119878310

[fin-w]

One test is failing when running cargo test and make test :

---- core::detect::tests::test_detect_with_options_with_filter_list_except stdout ----
thread 'core::detect::tests::test_detect_with_options_with_filter_list_except' panicked at src/core/detect.rs:148:9:
assertion `left == right` failed
  left: Cym
 right: Eng

I'm not sure what the correct fix is for this. I'll have a look at the test again and try to correct things though.

[fin-w]

It seems like I just had to filter out Welsh in the test to make it pass, so I've fixed that. Hopefully this is ready to merge now?