Create variable to ignore specified filetypes for highlight

I have a problem with the combination of this plugin and [unite.vim](https://github.com/Shougo/unite.vim).
Because unite.vim shows extra whitespace from the end of the line, the unite buffer becomes like [this](https://camo.githubusercontent.com/0b6b0d3c3e442faf0278351a38869d696ad7d35c/68747470733a2f2f662e636c6f75642e6769746875622e636f6d2f6173736574732f333937392f323336363231342f38396637363737632d613666352d313165332d383635332d3435323866653033663037662e706e67).
So I want this plugin not to highlight buffer with unite FileType.

And markdown needs extra whitespace to start a new line, I want to disable this highlight for buffer with mkd FileType too.

So I introduced `g:extra_whitespace_ignored_filetypes` option to solve such problems.