Add support for truncation length to buttercup-stack-frame-style

[doublep]

Currently, crop style always truncates lines at 79 characters. It would be nice to have this adjustable. Rationale: it is no longer 1980s and e.g. my console has ~260 characters per line. I usually use untruncated backtraces to avoid losing information (it is too often important in my experience). But non-truncated backtraces are notoriously hard to read. So I'd like to use a number reasonable for me (say, 250), without imposing it on anyone else. With 3x longer lines chances of stripping so much from the backtrace as to make it useless would be considerably lower.

I would also add support for this to Eldev. For ERT backend there is already variable ert-batch-backtrace-right-margin, but for Buttercup I wouldn't be able to achieve this without hacking.

Possible ways to implement:

• easy: just allow setting buttercup-stack-frame-style to an integer, anything non-positive being a synonym to full, a positive number working as crop, but with the specified line length;
• more generic: allow to specify styles with options, e.g. sth. like (crop :length 250), with (cdr style) being an arbitrary plist.