Add an ordinal_form property to Locale

[mbirtwell]

This exposes the rules for selecting ordinal forms in a similar way to the rules for selecting plural forms. In general a languages plural forms and ordinal forms might not be related.

I was hoping to provide a format_ordinal function to do conversions like 1->1st, 2->2nd, 11->11th, 23->23rd etc. unfortunately the CLDR doesn't have any convenient data for this. It does have some in the rbnf data but the language coverage at least in CLDR 26 seems patchy and pretty hard to interpret.

But I think this is useful on it's own because it provides access to the ordinal rules so that you could construct a simplified solution to a reduced problem based on these rules. Indeed that is what I'm doing for the small set of locales that I need to support.

[codecov-io]

Current coverage is 83.53%

Merging #270 into master will increase coverage by +0.01% as of 0db1ac4

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+ Hit           3184    3186     +2
  Partial          0       0       
  Missed         628     628       

Review entire Coverage Diff as of 0db1ac4

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[sils]

@mbirtwell apparently we have conflicts here and I'm afraid this needs a rebase

[akx]

Looks good to me.

As an aside, it would be nice to have some sort of table listing the CLDR features Babel currently supports (and probably a list of those it doesn't, too).

[sils]

I've filed #291 for the listing.