Split package up into multiple packages to be more modular

[neocotic]

This PR answers the question in #7. Yes, this package can be split up.

I've utilized Lerna to split this package into the following sub-packages within this repository (which will now be considered a "monorepo"):

• convert-svg-core
• convert-svg-test-helper
• convert-svg-to-png

I've done my best to update all documentation, however, this could probably do with some additional care in the future.

Unfortunately, and the reason that this is a breaking change (which I was really hoping to avoid), is that I've had to undo what was done in #9. In order to get this working the way I had imagined, I had to expose a pre-configured instance in the convert-svg-to-png package instead of a constructor. Unfortunate, but unavoidable, I believe, for now, at least.

I have the install setup perfectly (I believe), especially for contributors, but having never used lerna before, I am a bit scared of the publish mechanism, which appears to try to do a lot. So I might need to take care with that, especially for the first few times.

[neocotic]

I still need to rename this repository to the more generic convert-svg and update all references to it accordingly. Fortunately, GitHub redirects automatically, which will be good while we migrate over.

[neocotic]

I will also need to ensure that other services that we use are updated. For example:

• Node Security Platform
• Travis CI

[neocotic]

I've renamed this repository from convert-svg-to-png to convert-svg, updated all references to the repo URL, and updated NSP and Travis.