Replace coef argument in plotting functions with drop and keep arguments

[s3alfisc]

We should make use of regex-based coefficient selection whenever possible.

The coefplot() function and method currently support a coefficients argument - I think it makes sense to deprecate these in favor of drop, keep, and exact_match.

As _select_order_coefs will be a function not only used for etable but also confint and coefplot, maybe it makes sense to move it from summarize.py to dev_utils.py (which is the script in which I collect non-user facing development utilities)?

@Wenzhi-Ding, maybe something for you to pick up? =)