differentiate between locales en-us and en-gb

[georgpfolz]

• I have searched the issues of this repo and believe that this is not a duplicate.
• I have searched the documentation and believe that my question is not covered.

Feature Request

I love the localized formats! But the English outputs for en are US-American formats (concerning the order of day and month). It would be great to be able to specify en-gb to get more European results:

now = pendulum.now()
now.format('LLLL', locale='en')  → Thursday, September 4 1986 8:30 PM
now.format('LLLL', locale='en-us')  → Thursday, September 4 1986 8:30 PM
now.format('LLLL', locale='en-gb') → Thursday, 4 September 1986 8:30 PM

What I also miss is a localized format token that returns just the day and the month (maybe LL-?), so that it's easily combined with other tokens in a format string:

now.format('LL-', locale='en-gb') → 4 September
now.format('LL-', locale='en-us') → September 4
now.format('dddd, LL- LT', locale='en-gb') → Thursday, 4 September 8:30 PM

[Secrus]

Done in #731