API: adds weekday_common accessors (GH6936)
closes #6936

prob nice to add these for DatetimeIndex as well
 can you rebase this
 Two remarks:
- needs some docs + docstring (how is it different from `week`?)
- is it necessary that we have both? I don't know why we have now both `week` and `weekofyear`, but if it is mere historical reason we maybe should not repeat that? (but if this was done for convenience, then it is OK of course)
 In theory if `week` (I think `weekofyear` is just an alias) was a function, then this would be easier. But that just breaks everything. 
 To be clear, with my last point I meant having both `week_common` and `weekofyear_common`, instead of just choosing one of the two. And not about adding this one (although I don't really like the name, but I haven't directly a better idea)
 oh....then that's ok (I agree name needs some thought though)
 @trailsquirrel can you rebase this (and move release notes to v0.14.1)
 @trailsquirrel defering to 0.15.0. have to think about these names
 @jorisvandenbossche what did you think about these names?
 - I am not really liking the name `week_common`, as I think the ISO standard is maybe not always intuitive, but it is AFAIK the 'common' standard to give week numbers (eg used in business, governments, ..)
- For the more intuitive week number, why this convention and not another one?
  - I think this PR implements it as 1-7 january = week 1, 8-14 january = week2, irregardless of the day of the week?
  - another possibility would be to let each week start on Monday (or Sunday), but the week with firs tmonday (sunday) of the week- = week 1. And all days before that get week number 0. This is how `strftime/strptime` in the datetime module does it eg: https://docs.python.org/2/library/datetime.html#strftime-and-strptime-behavior (`%U` and `%W`).  
    Just to ask the question: is there a common definition for a non-ISO week number?
 closing as stale
