Add --find-interpreter option to build and publish commands

Some behaviors changed:

1. `maturin build` now implicitly translate to `maturin build -i python3`, meaning that we only build for the current python3 interpreter in `PATH`
2. `maturin build --find-interpreter` makes maturin search for interpreters on host, also search for bundled sysconfig when cross compiling
3. Mix `-i` and `--find-interpreter` is not allowed
4. `maturin publish` does the same as `maturin build`