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. 
Minor nitpick — but in cmus `if`s should be without extra spaces inside parentheses, like `if ((…))`. Although I understand that you are doing this for readability, consistency in the codebase is more important IMO.