Add documentation for usage of "extra_types"

[ollie-bell]

The parse function docstring mentions documentation for extra_types argument, but I cannot find it. Can it be added?

The default types are not sufficient for my needs. e.g. I would like to be able to enforce letters and numbers ONLY in a field of the format string (currently there is one for letters, numbers and underscores which doesn't work for me, as my format strings are all underscore delimited). I also have datetime strings of format %Y%m%d which doesn't seem to be supported for datetime parsing.