add new csv formatter

[takumakanari]

This pull request is adding new formatter named 'csv' to format log in csv(or tsv) style.

We can config like as follows:

format csv
fields name,age,message

We can output in also tsv style by setting 'delimiter':

format csv
delimiter \t
fields name,age,message

[tagomoris]

Code looks good.
But I wonder whether fluentd built-in formatter should have CSV formatter or not.
Of course, we can publish CSV formatter plugin on rubygems.

[repeatedly]

Could you use data approach instead of each assert in one test?

fluentd/test/plugin/test_filter_grep.rb

Line 109 in 1c70884

data(

[repeatedly]

I commented and others are looks OK.

I think CSV/TSV is popular and Ruby has CSV library in standard library.
So I think including CSV formatter seems no problem.

[takumakanari]

I fixed tests, could you review it again?