Add --cafile command line option.

[bnoordhuis]

R=@rvagg?

[rvagg]

taking a cb option that's only used when erroring is a bit gross, perhaps it'd closer to idiomatic if the thrown error bubbled up and the try/catch was done at the download() call?

[rvagg]

A bit smelly, I understand that it's hard to do much better without a more heavy refactor. Things I don't like:

• the use of cb only for errors, the fact that it has a cb argument at all in fact
• the fs.readFileSync() to read the cert file

A begrudging lgtm since I understand the challenges of doing a better job, I would prefer the cb thing be handled but if you feel strongly enough that the way you've implemented it is good enough then go ahead.

Yet another refactor for the future I suppose.

[bnoordhuis]

I added a commit that lets the exception bubble up. I'm a bit on the fence on whether it's an improvement; let me know what you think.

[rvagg]

I'm happy with the clarity of the changes, but generally unhappy with the structure of the call chain but that's a matter for another day! lgtm