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