update SVGO and other devDependencies

[johnd0e]

Alternative to #145

N.B.:

• happiness left heavily outdated
• svgtopng.js: unmodified sample code from https://github.com/domenic/svg2png/blob/master/README.md
  Although current svg2png already has cli support we still need helper buildscript
  because builtin cli lacks overwrite option (Feature request: Flag to overwrite existing file domenic/svg2png#100)
  In our current setup dist\images\toggle.png has to be overwritten during build process
• leaflet moved from devDependencies to peerDependancies as demanded by
  PLUGIN-GUIDE

[robpvn]

Hi, very nice of you to contribute this! I have a couple of issues with this PR, though.

• I don't see the need to update so many dependencies at once when all we want is a better svgo.
• Does updating svg2png and rewriting the way it's used get us any improvements?
  • It seems you forgot to add pn as a dependency. Looking at the Travis CI setup, we don't actually call build, only test, so it might not notice but I think build would fail.
• Raising the minimum for Leaflet from 1.0.3 to 1.4 is big breaking change which I won't do without a very compelling reason.

I guess all we really need to do is update svgo and not much else, the way I see it.

[johnd0e]

I don't see the need to update so many dependencies at once when all we want is a better svgo.

Well, I will try to explain my point.

1. There are 2 kinds of dependencies.

   • dependencies in production, where we should update carefully, because if we change our version requirements we can break other projects that use Leaflet-Minimap (as not every project will comply to that new version of our dependency).
   • devDependencies, where we can use whatever version that we want, and nobody else cares.

2. We have only 1 dependency of 1st type (and it's incorrectly defined in devDependencies).
   Technically leaflet 1.4 is fully compatible within minor version changes, but I agree that we can leave it at 1.0.3.

3. And main question: why we ever need to update any dependency? I have my answer:

   • Node community updates every package day by day making a lot of improvements. Some of them are subtle but still important.
   • Currently dependencies' versions defined in our package.json are extremely outdated. Try npm audit and you see that there are a lot of known vulnerabilities.

Does updating svg2png and rewriting the way it's used get us any improvements?

I believe that we should keep all dependencies in actual state. It's safe, and I do not see any drawback.

I've just updated this PR according to my vision.

It seems you forgot to add pn as a dependency.

It is dependency of svg2png, we have not define it explicitly.
Try to build and see.

[johnd0e]

leaflet moved from devDependencies to peerDependancies as demanded by
PLUGIN-GUIDE.