Parse out fields from /proc/meminfo

Newer kernels have changed the order of the lines in the /proc/meminfo
find (MemAvailable is now line 3). To support this and older kernels,
use the key from the map when getting data from it rather than static
line numbers.

Addresses #142, but is untested (fonts are refusing to load for some reason) beyond extracting the monitor logic into its own file and testing in ghci. Haskell makes me confident in it though ;) .