Update the check for fzf installed via homebrew

[danielnolan]

• The existing check for fzf installed via homebrew doesn't work for Apple silicon Macs, nor does it work for those who use homebrew on Linux.
• Add brew_fzf_path variable to get the brew prefixed path
• Update the isdirectory conditional to check this variable and use it for the Plug function call if it is a directory
• Closes #681

[mikegee]

Does this behave well when homebrew isn't installed or when fzf isn't installed via homebrew? (Perhaps the former is out of scope in this project.)

[danielnolan]

@mikegee I made a small change to ensure that brew is executable. If fzf isn't installed via homebrew Plug installs it to ~/.fzf https://github.com/thoughtbot/dotfiles/pull/713/files#diff-d38c94eb3c98eb2f3fbf6ced6c8e3c36a0135e5ad558a10d09efc83f31fc048aR29