Add shell completion functions

[patatahooligan]

Most shells have some directory in /usr/share where packages can place scripts to implement tab-completion functions. It would be nice to implement these functions and have the PKGBUILD place them where appropriate.

I can't say what the ultimate goal for the issue should be but I believe it should at least cover bash and zsh which are very popular.

[Kharacternyk]

v2.* doesn't have parametrized command line options. It may get them in the future, though, so I don't close this.