Added AbsoluteSourceFilter for changing relative src to Fully Qualified

This is useful if an image is root relative but should really be going through
a cdn, or if the content for the page assumes the host is known i.e. scraped
webpages and some RSS feeds.
Made AbsoluteSourceFilterTest exceptions include their filter class name.

This should help identify what contexts are required for the filter.
Test have also been updated to make sure exceptions are raised when
required contexts are missing and that the exception message includes
the name of the class where the required context is missing.
use #gsub instead of #gsub! in MarkdownFilter#initialize
Merge pull request #32 from jakedouglas/patch-1

use #gsub instead of #gsub! in MarkdownFilter#initialize
bump version to 0.0.6
Delete erroneous trailing {}s from README

I'm not familiar enough with this Gem but I'm guessing that {} used to
work in place of 'Hash.' It doesn't anymore, and this wasted a couple
minutes for me, so I'm fixing it here. I could be wrong though?
Merge pull request #34 from mrb/README_fix

Delete erroneous trailing {}s from README
remove github-linguist dependency. error out on attempt to use syntax filter w/out linguist
update readme with syntax highlighting instructions
changelog note about removing github-linguist
bump version to 0.0.7
Merge pull request #35 from jch/optional-linguist

Remove github-linguist dependency
Raise the roof---I mean, error
Merge pull request #37 from gjtorikian/raise-errors

Raise the roof---I mean, error
bump to 0.0.8
Don't monkeypatch Nokogiri in 1.9
Merge pull request #40 from jch/no-nokogiri-monkeypatch-in-19

Don't monkeypatch Nokogiri in 1.9
Merge remote-tracking branch 'benubois/relative'

Conflicts:
	README.md
Merge branch 'master' of https://github.com/jch/html-pipeline
bump escape_utils and linguist since it uses it too