Add (basic) optional Xinerama support

This commit adds optional Xinerama support (it's loosely based on https://github.com/llchan/bar/commit/9bfec1d118e9d302e1f14ac4978606fdf9aa8e7e#commitcomment-2249694)

A few of things to note:
1. The call to get the first X screen's geometry is now mostly redundant (with Xinerama support enabled) as the geometry can be calculated by summing the monitor widths.
2. This patch does not handle adding and removing monitors.
3. This patch does not deal with offset monitors.

By default, text spans the entire bar (as before). However, `\sn` will restrict text to display `n` (zero indexed) and `\s$` will restrict text to the last display.