Support separating digits with _ as in 1_000 and 100_000.42
Minor tests corrections.
Merge pull request #18 from fenollp/_-numbers

Support separating digits with _ as in 1_000 and 100_000.42
Update description.
-type specs from Erlang need work. Note they are grammatically unlike -specs.
A more understandable snippet, inspired from LYSE.