config: added config path override

The override uses the "WKLINE_CONFIG_PATH" environment variable to look
for config files.  If the config can't be opened it will report an error
and default to the xdg config.
widgets: wait for threads to join after cancelling
widgets: free resources when returning from init function
config: exit if unable to read config file from env var
Merge branch 'feature/conf-env-var' into develop
config: let env var define the entire config path, including filename

Ref #51
docs: add info about config env var
widgets: improve error handling and cleanup

Variables are initialized, pointers are validated before being freed,
most widgets now have a cleanup label which handles cleanup if any
errors occur during widget initialization.
widgets/magick: allow full configuration of any gradient overlay
widgets/magick: set blur radius = blur sigma value