`brew list` doesn't skip .txt versions of meta files

[adamv]

https://github.com/mxcl/homebrew/blob/master/Library/Homebrew/cmd/list.rb#L46

brew list skips meta files...but doesn't skip showing the .txt versions that install also scans for.

Other thoughts:

• Make the META_FILES list into a smart class with its own include? method
• Add README.* to metafiles, not just .md and .txt

[jacknagel]

Making it into a class might also help with #10141.

[adamv]

Rolling into #10141.