urxvt: Copy selection to clipboard and sync clipboard to selection.
urxvt: Force /bin/echo instead of builtin for clipboard.

By default, the system call uses sh(1) whose built-in echo command is
extremely limited. It does not understand the -ne flags and will just
print them to stdout.
install: Warn if xsel or autocutsel are not installed.
Merge branch 'improve_clipboard'

This branch enables any content in clipboard to be automatically synced
to the primary selection buffer. Also, anything selected in urxvt now
gets synced directly to the clipboard.