Config: Add Config and GlobalConfig classes

Config: Add Config and GlobalConfig classes

Allows configuration options to be fetched from context.

Only one implementation, GlobalConfig, is provided, which
simply returns $GLOBALS[$name]. There can be more classes
in the future, possibly a database-based one. For convinience
the "wg" prefix is automatically added.

Ironically, this adds the $wgConfigClass global variable
which is used to determine which implementation of Config
to use by default.

The ContextSource getConfig and setConfig methods were introduced
in I23194d1ba (1.23), but have no uses in Gerrit, so they can safely
be re-purposed.

Change-Id: I13baec0b6d4ea7badf20b9c5f9b40846348838e4