Update Homebrew instructions for Mojave and some other tweaks

[apjanke]

This PR revises the Homebrew installation instructions to update them for Mojave and newer versions of macOS, and do some other tweaks.

Changes:

• /usr/local/bin is on the $PATH by default now in newer versions of macOS. (It's in /etc/paths.) This PR edits the instructions to tell you to only do the path changes if you're on an older OS.
• /etc is the more typical path to use for fiddling with the /etc files, I think, not /private/etc. /etc is the same location that other Unixes use; I don't think there's any reason to expose users to the details of /private.
• $PATH is already exported; there's no need to add an export statement when you're changing its values
• Added the command-line technique for installing the Xcode CLT
• I changed the zsh instructions to say "in addition to ~/.bashrc" instead of "instead of ~/.bashrc". Even if your primary shell is zsh, you'll sometimes find yourself in bash (especially if you're doing scripting), so you want the Homebrewed stuff available in bash too, even if you're a Zsh-er (like me)
• A couple grammar tweaks
• The "for" is part of the official name for for the “Command Line Tools for Xcode”, so I changed the highlighting there.
• Added instructions for how to actually edit /etc/paths as admin

[apjanke]

Totally your call.

I have revised this PR to remove the nano instructions.

[apjanke]

...and squashed it to tidy up the commit history.