Added ability to parse numbers formatted with ', press '

[ddurdik]

I ran into a corner case where I needed to create a PhoneNumber object using the output of another PhoneNumber with the "{base_number}, press {extension}" format. This PR makes that possible

[va-andrew]

Very nice. As long as we're cleaning up extension separators, let's have:

BACKEND_EXTENSION_SEPARATOR = 'x'
VERBOSE_EXTENSION_SEPARATOR = ', press '

so there's a single definition for both kinds of separators?

[ddurdik]

Can do. I made the change in the form as well to keep things consistent

[va-andrew]

Just merged, will do a version update and PyPI release this week