Add cabal_sandbox_info function

Reports whether the current working directory is within a
sandbox. Useful to check before installing Cabal packages into the
global registry.
Fix URL to gi (gitignore) function

Previous URL doesn't seem to work because the host requires a www
prefix.
Merge pull request #2542 from puffnfresh/gitignore-plugin-url

Fix URL to gi (gitignore) function
Merge pull request #2541 from puffnfresh/cabal-sandboxes-info

Add cabal_sandbox_info function
Merge remote-tracking branch 'origin/master' into Darwin