Add conf.toJson() method

[ath88]

Resolves #406

It simply uses the implementation of toString, including the sensitivity-censoring. Also, it changes toString to use toJson to keep it more dry.

A test is added too.

[coveralls]

Coverage increased (+0.02%) to 93.225% when pulling db9247c on ath88:add-tojson-method into 9ca2d82 on mozilla:master.

[coveralls]

Coverage increased (+0.02%) to 93.225% when pulling 4ce7882 on ath88:add-tojson-method into 9ca2d82 on mozilla:master.

[SidSethi]

Thanks @ath88 for making this PR - would love to be able to use this function instead of what we currently do JSON.parse(conf.toString()

[ath88]

Any maintainers available for a quick gander at this menial PR? :)