Feat/optional codecs, HTJ2K support

[jmhmd]

Motivation

This PR reconfigures the image decompression codecs such that they can be included piecemeal or all at once. The use case is for applications that only handle one or several known transfer syntaxes, and these can only included the necessary codec and thus reduce the javascript bundle size. This also includes support for HTJ2K compressed images. Sorry about formatting changes in some of the Markdown files thanks to my editor auto-formatting.

Breaking changes

• Web worker configuration/initialization rolled in to main configure() function
• Paths to external codecs (other than big/little endian, RLE) must be included in the config as they are no longer rolled into the main javascript bundle. An allDecoders.js file is provided for convenience.
• Migration steps listed in docs/Migration.md

HTJ2K

This adds support for loading HTJ2K compressed images using the emscripten build from Chris Hafey here. See documentation in docs/HTJ2K.md. There may be other approaches to how to best load these as HTJ2K is not a standardized transfer syntax, and I am open to opinions.

Not sure if this should be a separate pull request, but I was working on both at the same time. I think it could pretty easily be removed and submitted separately if that's preferred.

[swederik]

Thanks! I have some codec work on my plate very soon so I'll try to find out how to get this stuff in with it. We are also interested in htj2k and adding codecs one by one so our goals are aligned :-)

[jmhmd]

That's great to hear! Looking forward to it. Happy to discuss any of these proposed changes or strategies as well.