add optional width and height for collapsed minimap

[edelater]

A small change to optionally set the height/width for the collapsed minimap.

I have a lot of leaflet controls on my maps, and it looks nicer if they all have the seem look-and-feel (tm).

[robpvn]

Seems straightforward enough, but how does the arrow icon look if you set very large or ver small collapsed sizes?

[edelater]

With larger sizes either:

• accept the default. i.e. a small arrow, and a really small preview of the map
• overwrite the css for .leaflet-control-minimap-toggle-display:

.leaflet-control-minimap-toggle-display {
    height: 26px;
    width: 26px;
    background-size: 26px;
}

Up to 50px still looks reasonable. beyond that size, the image should be changed as well.

For smaller sizes the css for .leaflet-control-minimap-toggle-display should be overwritten by the user.

It's also possible to have a larger collapsed size than expanded size. But I assume some sanity with the user :-)

[robpvn]

It seems that the change is not so great if it can lead to problems like that where things either look weird or you have to edit the CSS. (If you have to edit the CSS to make things work you may as well just edit all the CSS rather than have some of it as an option and some as manual edits...) So I think I'd rather want to revamp the icon in general to be SVG (as per issue #56) so that it can scale easily, and solve several problems at once.

[robpvn]

Actually it looks like I'll be using exactly your code for some of it, so to give credit where credit is due I'll reopen this PR. 👍