CLI app: refactor and improve performance

[bzz]

This is an ☂️ issue on improving CLI:

• migrate to using https://github.com/src-d/go-cli for flags and https://github.com/src-d/go-log
• add CPU profiler option
• split GetLanguage and GetLanguages out of common.go to language.go and languages.go
• address a TODO, avoid extra FS read \wo drop in accuracy