Add CLI support for YAML output

[mandrean]

Basically adds support for ./bin/swagger-jsdoc -o output.yaml.

[kalinchernev]

Hi @mandrean thanks for this suggestion!
I tested the feature and the code changes look fine, however the output file that I got was actually a json file with a yml extension.
The environment that I'm using for the test is regular cloud9 ubuntu machine:

[mandrean]

@kalinchernev Really? Wtf. It works fantastically well for us on a project... Need to investigate further.

[kalinchernev]

I was testing with .yml, not .yaml, which works well.

[mandrean]

Ah @kalinchernev! I should probably add .yml to the check as well then.

[kalinchernev]

It could be nice,yes :) Although not required, as the .yaml is the recommended one.

[mandrean]

@kalinchernev Done! :)

[kalinchernev]

@mandrean thank you!
@drGrove could you have a look if you have any other feedback on this one?
I consider this pull request ready to be merged.

[kalinchernev]

@drGrove @mandrean thanks!