Add available memory field to MemoryInfo
Add available memory field to MemoryInfo

On my system, `cat /proc/meminfo` yields:

```
MemTotal:       16289508 kB
MemFree:         1059928 kB
MemAvailable:    6774876 kB
Buffers:          954708 kB
Cached:          2824500 kB
SwapCached:           28 kB
...
```

Using the previously calculation of free memory `rest = free + buffer + cache`
yields 4839136 kB, which is substantially different to `MemAvailable`. I
wish to chart `(MemTotal - MemAvailable)/MemTotal` as a more accurate
statistic of how much RAM on my system is being used

teleshoes/taffybar