Improve config file handling

Allow ~/.invtool.conf as well, and parse all three config files if they
exist, rather than only using one.  Still fail if none exist.
update README to match config changes
write to LOCAL_CONFIG_FILE when being helpful
print all three config locations
no, always write to HOME_CONFIG_FILE
bump version to 4.1
Merge pull request #11 from djmitche/more-config

Improve config file handling