Some minor bugfixes and improvements

Features:

• Support stacked graphs
• [hwmontemp_linux] Provide name-based access to hwmon sensors via sysfs
• [mpd_all] Expose more informations and format time in [hh:]mm:ss

Fixes:

• Improve defaults and mechanism for data caching
• Escape XML entities in results by default
• [weather_all] Update NOAA link and use Awesome asynchronous API
• [mem_linux] Use MemAvailable to calculate free amount
• [mem_freebsd] Correct calculation and switch to swapinfo for swap
• [bat_freebsd] Add critical charging state
• [fs_all] Fix shell quoting of option arguments

Moreover, .luacheckrc was added and README.md was refomatted for the ease of development.