Use emconfigure and emmake wrappers for emscripten targets

Fix a discrepancy between documentation and implementation

Clarify and improve the documentation and bump to 0.2.0 to signal the
change.

Fixes lu-zero#13

Version bump

Thanks to Martin Larralde and Nolan Darilek the following features are
available:

- enable/disable static and shared library creation
- `LDFLAGS/CFLAGS/CXXFLAGS` support
- custom `make` targets

New release

Thanks to Owen Nelson we have the following features:
- `reconf` to trigger autoreconfig (was documented but missing)
- `make_args` to add additional make parameters (new)

Initial version

Missing:
- [ ] Cross compilation support
- [ ] Option override (e.g. `enable(foo, None).disable(foo, None)`)
- [ ] Easy pkg-config chaining for multiple dependencies
- [ ] Nonstandard options (not --enable and --disable)
- [ ] Option Compatibility probe
  - [ ] Mapping profile to --dis/enable-debug when available
  - [ ] Not hardwire --disable-shared --enable-static