Add external download command (#65).
Added download-command to manual page (#65).
Save settings in hash map.
Merge branch 'local/global-settings' into feature/downloader

Conflicts:
	src/main.h
	src/setting.c
Inform external download about used proxy.
Used g_slice_new instead of g_new.
Added new hint mode ;x (#65).

This hint modes calls a user defined vimb command if a hint was fired. The
command can be change and set via ':set x-hint-command' on runtime.
Removed missed function declaration in header file.
Fixed wrong return value for toggled settings.
Fixed wrong placed dependency include in Makefile.

Allow to generate dependency files during compilation with CFLAGS="-MMD".