Add ability to set an angle correction

[daynin]

Hello, @bbecquet! Your plugin is very useful but It has no way to set angle correction in some specific cases.

For example, I have an image like following:

If I create a marker with following settings:

L.Symbol.marker({
  markerOptions: {
     icon: L.icon({
       iconSize: [50,15],
       iconUrl: './some-path/arrow-up.png'
     })
  }
});

I will get something like this:

If I set "rotate: true", I'll get following:

But I want to get something like this:

In this PR I've already implemented a small addition to "L.Symbol.marker" API. Now we can set "angleCorrection: 90" (for example) to correct marker's angle.

L.Symbol.marker({
  markerOptions: {
     rotate: true,
     angleCorrection: 90,
     icon: L.icon({
       iconSize: [50,15],
       iconUrl: './some-path/arrow-up.png'
     })
  }
});

I think it will be useful not only for me.

[bbecquet]

Thanks @daynin! Indeed that's a useful feature, as it doesn't require to have the image file in the correct direction.

I'll merge :)

[daynin]

@bbecquet , thank you!