Add fish and elvish completions

[TymekDev]

clap_complete already supports both fish and elvish shells. This pull request adds completions for both of these.

[gaborcsardi]

Thanks, looks great! To include the completions in the tar.gz/deb/rpm packages, you need to copy them to build in the Makefile, see here and below:

rig/Makefile

Line 56 in 74bb252

mkdir -p build/share/zsh/site-functions

[TymekDev]

Done!

[gaborcsardi]

Great, thank you!