Replacing RedCarpet w/HTML::Pipeline for parsing.

HTML::Pipeline *in its README* talks about how to parse fenced code
blocks, at-mentions, sanitisation, and all sorts of other Good and
Useful Stuff that we'd have to reinvent adding on top of RedCarpet.

First up: an exploration spec to fully kick the tires and document the
API features we (will) use, in such a way that future breaking API
changes break both our main-app code *and* the exploration spec.
(Because if only our main-app code breaks, it *could* be a bug in *our*
code…).

This commit is the first "tire kicked" in the exploration spec.

RSpec: 691 examples, 0 failures
RuboCop: 179 files inspected, no offenses detected
More HTML::Pipeline exploration

Covering:

* a SanitizationFIlter
* a PlainTextInputFilter combined with a SanitizationFilter
* an HttpsFilter (combined with a MarkdownFilter)

RSpec: 696 examples, 0 failures
RuboCop: 179 files inspected, no offenses detected
Added specs for a MentionFilter pipeline.

RSpec: 698 examples, 0 failures
RuboCop: 179 files inspected, no offenses detected
Added specs for EmojiFilter.

Note, however, that one spec is commented out; lines 209-211 should
verify that enclosing an emoji within a `<tt></tt>` tag pair should
*not* expand the emoji, following the example of [this
spec](https://github.com/jch/html-pipeline/blob/master/test/html/pipelin
e/emoji_filter_test.rb#L38-L43) in the current HTML::Pipeline `master`
branch. However, uncommenting that spec will cause it to fail. Further
investigation?

RSpec: 703 examples, 0 failures, 1 pending
RuboCop: 179 files inspected, no offenses detected
Fixed Git confusion on previous commit. Ptui!

RSpec: 703 examples, 0 failures, 1 pending
RuboCop: 179 files inspected, no offenses detected
Using latest pipeline from GitHub fixes :tt bug.

The README for `jch/html-pipeline` states that the EmojiFilter does NOT
interpret emoji enclosed within `<pre>`, `<code>` or `<tt>` tag pairs.
The current released Gem on RubyGems, contrary to doc, ignores `<tt>`.
Current GitHub commit works as documented.

RSpec: 703 examples, 0 failures
RuboCop: 179 files inspected, no offenses detected
Spec for SyntaxHiglightFilter pipeline.

Note the nearly illegible `#gfm_highlighted_fenced_code_block_html`
method used for matching. If anybody has a better idea, I'd gladly
entertain a pull request.

RSpec: 705 examples, 0 failures
RuboCop: 179 files inspected, no offenses detected
MarkdownHtmlConverter spec changed.

Rouge syntax highlighting changed its containing markup format, which
broke specs that were not expecting the change. Nailing that down had
me chasing my tail for an *extended* period of time; now that I (think
I) know what to look for, I can go and re-enable the exploration spec.
Pffft.

Also added new `.rubocop.yml` file to disable new, intrusive cop. See
comments in that file.

RSpec: 705 examples, 0 failures
RuboCop: 179 files inspected, no offenses detected
RedCarpet explore spec now handles updated Rouge.

RSpec: 705 examples, 0 failures
RuboCop: 179 files inspected, no offenses detected
Added previously-forgotten AutolinkFilter.

RSpec: 707 examples, 0 failures
RuboCop: 179 files inspected, no offenses detected
Simple cleanup of MarkdownHtmlConverter spec.

RSpec: 707 examples, 0 failures
RuboCop: 179 files inspected, no offenses detected
Refactoring MarkdownHtmlConverter.

Isolating knowledge of RedCarpet(+Rouge) API in internal module, before
developing *parallel* module using new HTML::Pipeline code.

RSpec: 707 examples, 0 failures
RuboCop: 181 files inspected, no offenses detected
More MarkdownHtmlConverter evolution.

Tightened up `RedCarpetConverter` module; better example now for the
newly-forming `HtmlPipelineConverter`.

RSpec: 707 examples, 0 failures
RuboCop: 181 files inspected, no offenses detected
Using (only) HTML::Pipeline for Markdown rendering

Plastering some overly fragile specs and support code along the way.

RSpec: 708 examples, 0 failures
RuboCop: 181 files inspected, no offenses detected
Merge pull request #140 from jdickey/issue-136

[Fix #136] Replacing RedCarpet w/HTML::Pipeline for Markdown content parsing.