Add a format flag to extract function names from named scopes

[Andrey Semashev]

The named scope formatter can have a special format placeholder which would try to guess a function name from the scope name. If the scope name matches the conventional "type func(args)" pattern, the placeholder would only extract the "func" part. Otherwise the placeholder should expand in the whole scope name (similar to %n).

[anonymous]

This would be good. Just what i'm looking for. Also file name without path would be useful, as at least on visual studio c++ the path is also included. So log files end up full off clutter.

So instead of getting verbose formatting like this

"int cdecl main(int,char *[]);c:\users\someone\documents\visual studio 2013\projects\source1.cpp;201"

one could get concise formatting like this:

"main;source1.cpp;201"

[Andrey Semashev]

Implemented preliminary version in develop: ​https://github.com/boostorg/log/commit/15ed06e6065e3b83ecd2199562b7b6d2591f5cbb.

[Andrey Semashev]

Merged to master.