Support MacOS

[montaguegabe]

This issue fixes #23, a problem apparently appearing on MacOS 10.8 and above. The diff command has been changed as well on Mac. Added instructions on how to install the matching grep and diff commands, and modified the code to use them.

[sp1thas]

I propose to add these brew install commands directly into the utils/install.sh file.

[montaguegabe]

Sounds good - I forgot that users of dropboxignore are pretty much guaranteed to be developers so they will definitely have homebrew

[sp1thas]

Thank you @montaguegabe for your contribution. Your documentation-related changes should also be applied in docs/installation.md (I know... duplicated docs.. we should find a better approach on this)

[montaguegabe]

I can update so that the brew commands are part of the install script and modify the docs in both places

[montaguegabe]

One thing to note actually: at least on my Mac running a brew install command will automatically update homebrew, which can sometimes take up to ~3 minutes of somewhat verbose output (maybe even longer if it hasn't been updated in a while). An unwary user might think all this heavy lifting is to install dropboxignore. But I guess we can circumvent the auto-update with the HOMEBREW_NO_AUTO_UPDATE=1 prefix so maybe I'll do that. I'm not exactly sure what the rationale is behind Homebrew trying to update each usage

[montaguegabe]

Sorry don't merge this yet - I hadn't tested install.sh and brew doesn't work with sudo

[montaguegabe]

Ok all set! I figured out how to drop sudo for the brew commands as per the second answer on https://unix.stackexchange.com/questions/353206/is-there-a-reverse-sudo (I actually tested a mock brew command to make sure it wasn't secretly executing with root permissions). Also added a fix for the few people who have brew install packages to a different location. Have tested the last commit: both the install script and the installed command itself.

[sp1thas]

LGTM! Thanks :)