main: add command line arguments for overriding user config

Added overrides for the following config options:

- monitor
- position
- height
- theme_uri

Closes #50
main: use strtol for converting int args
Merge branch 'feature/getopt' into develop
docs: update man pages

Ref #46, #50
widgets/desktops: implement reading of desktop names

This commit also includes various improvements in memory and error handling
when fetching desktop information.
widgets/weather: use dynamically allocated memory for query strings
widgets/weather: handle weather code and temp conversion more safely