Add the completion for bash

[umireon]

This bash completion will be helpful for those who are lazy to type or who could not remember the version number of the latest Lua distribution as seen in #22.

$ . ~/.luaver/completion/luaver.bash
$ luaver[TAB]
current                 list-luarocks           unset-default
help                    set-default             unset-default-luajit
install                 set-default-luajit      unset-default-luarocks
install-luajit          set-default-luarocks    use
install-luarocks        uninstall               use-luajit
list                    uninstall-luajit        use-luarocks
list-luajit             uninstall-luarocks      version
$ luaver install 5.[TAB]
5.0    5.0.2  5.1    5.1.2  5.1.4  5.2.0  5.2.2  5.2.4  5.3.1  5.3.3
5.0.1  5.0.3  5.1.1  5.1.3  5.1.5  5.2.1  5.2.3  5.3.0  5.3.2
$ luaver install 5.1.[TAB]
5.1.1  5.1.2  5.1.3  5.1.4  5.1.5

Future Improvement (TODO)

• Completions with install command are painfully slow.
• list commands should be modified for machine readability.