Add conditionals in format strings

Idea from #150. Syntax `%{if COND then A [else B] }`. 
I used `expr.c` for parsing. `format_print.c` was modified. It uses `gbuf` now: I think, it is more convenient.
Id's in `COND` can be options. Any unknown id is `false`. Id `stream` checks if `is_http_url( %{filename} )`.
I haven't changed/removed/added default formats yet. 
I am probably missing something: this is a section in `Filters` chapter. Filters already support *filename*, *artist*, *albumartist*, *album* etc. — so what is this `Keys` section is about, and what is the difference between `Keys` and previous sections?