Remove dep on umaintained difflib

[SamWhited]

This removes the testing dependency on github.com/pmezard/go-difflib which, per the README, is no longer maintained.

I did not try to match the diff output exactly, assuming these weren't actual patches being applied somewhere, but the relevant information is still included.

A failing test now looks like this:

blackfriday (v2 *=) $ go test
--- FAIL: TestReference_EXTENSION_NO_EMPTY_LINE_BEFORE_BLOCK (8.09s)
    helpers_test.go:173:
        file: Amps and angle encoding

        line: 6

        -<p>4 &lt; 6.</p>
        +<p>4 &lt; 5.</p>

        line: 16

        -<p>Here's an inline <a href="/script?foo=1&amp;bar=2">link</a></p>
        +<p>Here's an inline <a href="/script?foo=1&amp;bar=2">link</a>.</p>
--- FAIL: TestReference (7.16s)
    helpers_test.go:173:
        file: Amps and angle encoding

        line: 6

        -<p>4 &lt; 6.</p>
        +<p>4 &lt; 5.</p>

        line: 16

        -<p>Here's an inline <a href="/script?foo=1&amp;bar=2">link</a></p>
        +<p>Here's an inline <a href="/script?foo=1&amp;bar=2">link</a>.</p>
FAIL
exit status 1
FAIL    github.com/russross/blackfriday/v2      8.318s

[SamWhited]

Rebased on top of #509

[rtfb]

Thanks, Sam! It's great that we got away with so few lines of extra code instead of this dependency.