Add --ignore-system-yangs option

[immibis]

If this option is given then pyang will ignore all the default YANG paths and only use the paths specified on the command line.

[coveralls]

Coverage increased (+3.6%) to 39.927% when pulling d6c0684 on immibis:ignore-system-yangs into 458a50f on mbj4668:master.

[coveralls]

Coverage remained the same at 36.374% when pulling d6c0684 on immibis:ignore-system-yangs into 458a50f on mbj4668:master.

[immibis]

Looks like there's a CI setup problem, the installed version of pytest isn't compatible with Python 3.3

[mbj4668]

Hi,

This seems useful. I suggest you rename the option to "--ignore-default-path", and update the documentation pyang.1.dbk with this new option (and update the text re. --path)