Captured-log support for --show-capture

[twmr]

In #3176 a --show-capture option was added (part of the reporting section
of the pytest -h helptext), which currently supports a discrete set of
values: {no,stdout,stderr,both}.

I propose to add support for captured logs to --show-capture. This idea
came up in #3204. The list of allowed values should probably be changed to
{no,stdout,stderr,log,all}. The show-caputure option should not only be
used in the report generation part of pytest but also when pytest enters
the debugger when --pdb is used (see #3186 and #3210).