Webassembly/emscripten build

[gabonator]

No description provided.

[gabonator]

This PR adds support for building webassembly (with emscripten). In the build folder it will produce sample HTML + JS file which parses some simple protocol and dumps the JSON report into console. Having RTL433 webassembly build allows integration this powerful decoder into web based applications. For example this one:
https://rawgit.valky.eu/gabonator/LA104/master/system/apps/79_cc1101osc/web/editor/index.html

[zuckschwerdt]

Thanks, interesting! I have commited the fixes to secplus_v1 and output_rtltcp with e826a46, you can undo those from this PR.

[zuckschwerdt]

I dug up an old idea to make the decoders easier to use stand-alone in #2667 -- you then wouldn't need to handle data_t but could build a output with simpler functions.

[gabonator]

Noticed some instabilities when analysis was executed more times, I will try to fix it and then reopen

[gabonator]

Demo with latest rtl433 decoder:
https://rawgit.valky.eu/gabonator/LA104/master/system/apps/79_cc1101osc/web/editor/index.html#decode_init;signal_nexus1

Or with custom signal passed in url:
https://rawgit.valky.eu/gabonator/LA104/master/system/apps/79_cc1101osc/web/editor/index.html#decode_init;500,960,480,980,480,1000,460,980,500,1000,460,1000,480,1000,460,1000,1440,1000,460,1000,1440,980,1440,960,520,960,500,1000,1420,1000,480,960,500,1000,480,1000,1420,960,1460,1000,1460,980,1420,1000,1440,1020,1440,940,520,980,1440,960,500,940,540,1000,460,1000,480,1000,440,1000,1460,1000,460,980,460,1000,480,1000,500,960,480,1000,460,1000,480,980,480,1000,1420,1000,1460,1000,1420,1000,480,1000,460,1000,440,1000,480,1000,1460