Support pattern start with character classes

[okkez]

In prevous version, following pattern raises syntax error:

<filter>
  @type grep
  <regexp>
    key price
    pattern [1-9]\d*
  </regexp>
</filter>

This example is derrived from filter-grep

[okkez]

On the other hand, we want to use regular expression in configuration file sometimes.
So, should we support config_param :pattern, :regexp ?

[repeatedly]

all tests failed.

[okkez]

Sorry, my fault...

[repeatedly]

Thanks!