feat: output type only accept IO or Array(IO)

- idea from branch v0.1
- use subclass of Process::Status as Err type

feat: use more simple ok? instead of success?

- simple means less error

feat: Exec.run return Exec::Status type

- Exec::Status has getter stdout stderr

feat: use Err type rather than a tuple

- No need override IO::MultiWriter IO type

fix: IO::MultiWriter not add STDOUT twice

refactor: Exec.run return Result Type now

- With Exec::IO::MultiWriter we can print and get output both
- Improve Exec.each_line performance
- Update CI, change spec option
- Update README

fix: add chomp option for output method

feat: add exec lib for crystal