Czech and Slovak algorithms.

[gaboull]

No description provided.

[ojwb]

The 2 and 3 letter codes should be those specified by ISO 639:

https://en.wikipedia.org/wiki/List_of_ISO_639-1_codes

So for Slovak that should be: slovak,sk,slk,slo (those for Czech above are also wrong but I've opened #151 to merge the Czech stemmer since I can write that one up for the website).

[ojwb]

The process for submitting a new stemmer is documented in CONTRIBUTING.rst. In particular we need a test vocabulary adding to snowball-data so there's test coverage and a page adding to the website with some background on the algorithm to aid future maintenance (if a bug is reported and all we have is the snowball implementation it can be hard to tell if it's an intentional design trade-off or an oversight).

The czech stemmer is already on the website so I know that it comes from a paper and who implemented it, so I can easily fill that in and I've created a test vocabulary from wikipedia data (in #151).

I don't know any background to the slovak algorithm here though.

[jimregan]

I'm not sure if this is necessary: the stemmers usually received lower-cased input, no? And unicode-aware case folding generally does the right thing with Slovak.