Add ":highlight clear" to reset to builtin colors

This balances change in resetting color scheme to default one.
Useful for building color schemes that are partially based on the
builtin one.
Add "x" flag to 'cpoptions' option

It makes vifm prefer to perform file-system operations with external
applications on rather then with system calls.  {EXPERIMENTAL},
{WORK-IN-PROGRESS}.  The flag will be eventually removed.  Mostly
*nix-like systems are affected.
Add 'syscalls' instead of "f" to 'cpoptions'

This one is safer as someone might have:

    set cpoptions=

in vifmrc, which will automatically use system calls, which are not
properly tested yet.
Update URL to Vifm's home page

From http://vifm.sourceforge.net (http://vifm.sf.net) to
http://vifm.info.

Also add link to Wiki.