Add config setting to skip sync pairs

[sciurius]

I would like to have a setting "skip" (boolean) in the pair definition.

Semantics: if "skip" is true, do not process this pair by default. To process this pair, it must be explicitly designated on the command line.

Rationale: I have several servers and vdirs that I sync. I would like to have all the settings in a single config file, but I want some servers/vdirs to not sync unless I explicitly ask to.