Add all languages supported by Snowball

Together with a simple change I'm going to propose to Stemmers.jl,
this allows specifying the language of a document and to use
the correct stemmer. In the long term, we likely need a mechanism
to create the types based on an exhaustive list. For reference,
I have some code for the R package ISOcodes which downloads the
ISO 639-3 list from Wikipedia. Likewise, lists of stopwords could
be downloaded from the Snowball project.

In the future I think isocode() could get an additional argument
to choose between alpha-2, alpha-3/B and alpha-3/T codes, but I
wasn't sure how to implement this kind of enum argument.

The name() function is already used by TextAnalysis.jl. If you
think this name is the right choice, I can modify TextAnalysis.jl
to import it for Language.jl.