--vimgrep option added, reporting every match on the line, like :vimgrep /pattern/g
Merge pull request #504 from inside/vimgrep

--vimgrep option added, reporting every match on the line
Remove unnecessary goto
Completely ignore stdin when searching given path

Piping into ag used to turn off the pretty output format, even if files
provided via command line argument were searched.
Allow stdin redirection

If `ag foo <file` is used, try to search the file like a file and not
like a stream. This has better performance and supports multiline
regexes. Fall back to stream behaviour if the system doesn't support
`/dev/stdin` or `/dev/fd/0` as this is still better than not supporting
it at all.