Add a parser type `text` to WebsiteAgent.

Sometimes you need to parse a document that has no structure or is simply not HTML/XML or JSON.
This can be useful to check updates on a plain text, source file, CSV/TSV, etc.
Rather than having our own default that diverges from ruby's, I'd like to put a note about the nature of /./ making use of this example and put a pointer to documentation like [this](http://ruby-doc.org/core/doc/regexp_rdoc.html).  How does it sound?