Add filter_grep plugin

`grep` is a general and useful plugin so I ported fluent-plugin-grep as a filter.

We don't have an enough experience for writing Filter test driver so
I wrote tests without test driver. After added several filters,
I will implement test driver for Filter.

@sonots Could you check this PR?
I removed obsoleted parameters.
And I noticed `replace_invalid_sequence` parameter is not used.
Can we remove this parameter?

May we use `string-scrub`?
https://rubygems.org/gems/string-scrub