Add +over support

[EduardTrutsyk]

I need to allow longitude output outside -180 to 180 range. This can be done by: +proj=merc +over +lon_0=0 +k=1 +x_0=0 +y_0=0 +datum=WGS84 +units=m +no_defs.

See the proj.4 docs on that. It seems +over is not supported by proj4js.

Is there a chance that this feature could be added? It would be very useful in dealing with certain scientific datasets.

[malaretv]

This would be useful for me too.

Happy to put together a pull request if the maintainers are willing to accept. I imagine it's basically adding a flag on the adjust_lon function that doesn't apply it if the over flag is present