Update: add "options" to the Georaster constructor

[ajgrowney]

Allow applications to forward custom headers to URLs (e.g. Authentication) when parsing georaster

[DanielJDufour]

Looks great! Thank you so much!

P.S. Just a heads up that I'll probably keep these additional options undocumented at this time. I'm committed to maintaining backwards compatibility for all the public documented georaster API (even across the next major upgrade). And it might be nice to change the options object in the future. It's not necessary now, but once georaster adds support for ascii grid and other raster formats, perhaps explicitly defining geotiff.js options vs. other options might be nice.

P.P.S. Awesome! I didn't know geotiff.js supported passing along headers until you submitted the PR!