Added the option to disable rules.

[jGleitz]

I added an option to disable rules. This may for example be wished to use a plugin like markdown-ins-del or to simply re-define syntax elements.

I added two tests for the functionality and will update the wiki if this gets merged.

As markdown-it enables all rules per default, I did not see a reason to include an enable option, too.

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This change is

[coveralls]

Coverage remained the same at 100.0% when pulling 9eae3ce on jGleitz:master into d5b61e2 on celsomiranda:master.

[curbengh]

#123