* partial implementation for emscripten
this pull request contain a partial but working implementation of emscripten backend
some implementations may be controversial.
here some implementation detail:
* cursor state:
* on grab: emscripten request pointer lock deferred and also set a callback when pointer lock change
the callback request pointer lock deferred.
* on hide: `emscripten_hide_mouse` exist but not `emscripten_show_mouse`
a pull request has been open on october 2016 but never been merged
so I copied the javascript function and put it in emscripten_asm_const function
* fullscreen: if fullscreen is requested then it request fullscreen deferred and set a callback on fullscreen change
the callback request fullscreen deferred
* run forever: this method use emscripten main loop to run an infinite loop
* keyboard callback doesn't consume the event. I think it is more apopriate as in desktop environment it is the same, is it ?
* emscripten dir is added in example and contains html pages
Some things that are not implemented:
* lots of events
* min and max dimension can be implemented with a callback that listen to size change and resize if dimension out of bound
* title may be implemented using javascript to change document.title
* Use std::os::raw in the emscripten bindings
* Fix emscripten code
* Update code
* Add CI
* Remove the emscripten-specific examples
* Add some information to the README
