* contains methods designed to be run once.
* is able to upgrade the datastore or the configuration.
* is based on methods names, stored in a text file with ';' separator (updates.txt).
* begins with existing function 'mergeDeprecatedConfigFile()' (options.php).
Isolate functions related to config in Config.php + add unit tests + code_sniffer.
options.php is not supported anymore, but its content will be automatically saved into config.php
Fixes #shaarli/Shaarli#41
*TODO*: update [documentation](https://github.com/shaarli/Shaarli/wiki#configuration).