support MMM format element in format_datetime function

[Javier Canizalez]

In the format_datetime function, add support to use MMM format element to get the Month name

let dt = datetime(2017-01-29 09:00:05);
print 
v1=format_datetime(dt,'MMM')

should print

January

[Alexander Sloutsky]

For feature addition, please use https://aka.ms/adx.uservoice (this repo is for KQL parsing library)