Allow a common repo as an argument to fresh-options

[parameme]

As the fresh ecosystem grows we may source a lot more (trusted?) upstream files. Perhaps we might consider supporting a common repository target as an option to fresh-options.

Example usage

# Default the repo for the following lines
fresh-options --repo=twe4ked/dotfiles --file=~/.zshrc --marker  
  fresh zsh/completion.zsh

  # Preserve access to local context by overriding repo to empty
  fresh zsh/my_completions.zsh --repo='' 

  # Honour the original syntax for different (non-local) repos
  fresh freshshell/fresh contrib/source-build.sh  

  # ...

  fresh zsh/prompt.zsh --ref=7192f1a
fresh-options