Install nim's tab-completion files

[nrclark]

Nim comes with Bash tab-completion scripts. For a given version of Nim (1.4.0 on my machine), these wind up in $HOME/.choosenim/toolchains/nim-1.4.0/tools/nim.bash-completion and $HOME/.choosenim/toolchains/nim-1.4.0/dist/nimble/nimble.bash-completion. It would be great if choosenim could also install these in a version-agnostic way, so that we'd be able to access them without having to dive into choosenim's internal storage.