Request for custom PhantomJS path

[cyberwombat]

I use svg2png in AWS Lambdas (which do not execute npm install so the entire node_modules tree must be uploaded. This causes issues because of phantomjs-prebuild. In previous versions of svg2png I was able to overwrite the binary since it just had it downloaded but with this it becomes a bit messy.

I have forked the repo and added/modified two lines to allow for setting a path to the desired PJ binary:

https://github.com/cyberwombat/svg2png/blob/master/lib/svg2png.js#L6-L7

I can then easily add this env path to the Lambda settings.

I suppose this can be improved and not have phantomjs-prebuild used in some cases but at least this addresses the Lambda issue. Would you consider adding this (or a PR) or is this too case specific?