Working with substring.

I developed a emacs plugin called [etags-grep](http://github.com/vmfhrmfoaj/etags-grep/tree/develop) for that select the tag among multiple tags, like [etags-select](https://github.com/emacsmirror/etags-select).
The etags-grep aim to reuse the codes of grep-mode specifically.

The TAG file have not include string of the exact line, As follows.
>Source file:  def command(self, cmd, params=()):
>TAG file:  def command(

I need to handle the substring.
So, I implement a feature that handle the substring.
Please, merge this patches.