Add option to use manual account details

Add command-line option --manual-account, which instructs to use details
present in config.json rather than use Gnome Online Accounts.
Add Help string
Move local excludes file to .config

* Move local excludes files to ~/.config/gnome-shell-google-calendar
* Gather config file locations into config.py
Updated README.md
Updated Help String

* Read config key 'manual' from config.json and if true use manual
settings from config file.
* Update Help String relecting the changes