look for Brewfile in HOMEBREW_USER_CONFIG_HOME

[rrotter]

Indirectly support XDG_CONFIG_HOME by leaning on brew's support for it.

when --global is set, we'll get the brewfile from:

• $HOMEBREW_BUNDLE_FILE_GLOBAL if set
• $XDG_CONFIG_HOME/homebrew/Brewfile (if XDG_CONFIG_HOME is set and file exists)
• $HOME/.homebrew/Brewfile (if XDG_CONFIG_HOME is not set, and file exists)
• ~/.Brewfile (unconditional fallback, but if this doesn't exist either, we should probably use $HOMEBREW_USER_CONFIG_HOME/Brewfile as the new default)

Starting this as a draft because:

• looking for feedback on whether this change is welcome at all (I suspect so, per: Global Brewfile would be better placed somewhere under ~/.config or ~/.local as per XDG #1002 (comment))
• looking for feedback on what the fallback default should be if no file exists
• I still need to add tests. I'll need to add fixtures for a home dir, since currently tests are using the user's real $HOME, which we don't want to read or write from in rspec.
• need to update readme and brew bundle --help text

[MikeMcQuaid]

• $HOME/.homebrew/Brewfile (if XDG_CONFIG_HOME is not set, and file exists)
• but if this doesn't exist either, we should probably use $HOMEBREW_USER_CONFIG_HOME/Brewfile as the new default

This is the only part I'm 👎🏻 on, I don't see any users wanting this.

• looking for feedback on what the fallback default should be if no file exists

The current location.

[MikeMcQuaid]

Code mostly makes sense so far. I'd be 👍🏻 on supporting behaviour change if XDG_CONFIG_HOME is set and otherwise I'd like to leave everything as-is. Thanks!