Add the bucket-to-explicit/select subcommands, support angular similarity

• Add bucket-to-explicit. This subcommand converts a finalfusion/fastText embedding file with a bucket-based n-gram vocabulary to an explicit n-gram vocabulary. (by @sebpuetz)
• Add an --similarity option to the similar/analogy subcommands. This option allows selecting the similarity measure. Cosine similarity is still the default, but angular can be used to switch to angular similarity. (by @danieldk)
• Add select subcommand. This subcommand creates a new embedding matrix from an existing embedding matrix that covers the given set of words. (by @danieldk)