Config file: Add ability to mask links

Adds the ability to mask links via the config file.

• Link mode settings are now optional (i.e. so that a mode is not required for a masked link), but an exception should be thrown if an unmasked link does not have a mode set, either via the default setting or an individual link settings entry.
• If a particular link has a mask setting specified both in the config file and via the command line, the command line setting should take precedence.

Fixes #22 (closed).