Add simple capitalisation functions

capitalise for first letter of sentence, capitalise_words for first letter of
every word.

capitalise_words is dumb but making it clever (avoid capitalisation of words sur as a, in, of, for, etc.) is language specific.

The functions are very simple and it could be argued that anybody should be able to reproduce them easily. However, it is handy to have them; in particular, when developing other code/package that has nothing to do with string manipulation, it makes it cleaner to not have these stray functions float around.