acmego: allow arbitrary rc filters enclosed in [[ ]]

[jacobvosmaer]

I would like to automatically run clang-format on some but not all C files I edit. Not every project uses it and I don't want to mess up an entire file while making a one-line change.

This commit adds a new [[filter]] syntax for the tag. In my case I would write [[clang-format]] into the tag of my C file and then when I hit Put, acmego runs rc -c 'clang-format $1' /path/to/file.c.

To avoid conflicts with Edit, we ignore [[ if it occurs after Edit.

I don't know if this commit is the ideal design but I think acmego ought to be able to do something like this. It would elevate it to the same level of flexibility as Watch.

[jacobvosmaer]

There was an issue where we could only read the first line of the tag, because of looking it up via acme/index. Added support for getting the tag via acme.Win.Info().

I also fixed a usability quirk: the first iteration required an explicit $1 in the filter to read from a file. By adding some rc glue, the filter now gets the file on stdin.

[jacobvosmaer]

OK so I learned that having the filter read from stdin is not practical: it prevents clang-format from finding a local .clang-format file. By referring to the file by name, the filter can see what directory the file is in.

[jacobvosmaer]

Closing in favor of #112. That one is much nicer to work with.