Use a code-span filter when finding the end of a paragraph

This is required because HTML tags occurring within code-spans
should not be taken into account when we make use of a HTML parser
in finding the end of a paragraph.

For example:

    Para ends here `<pre>`

    It shouldn't extend till here `</pre>`

Ref: https://github.com/vfmd/vfmd-spec/issues/1
Procedure for a code-span filter

This procedure does not worry about unclosed code-spans in order to
avoid potentially paradoxical situations like this:

    Before blank line `<pre>

    After blank line </pre>`

where if we consider the paragraph to end before the blank line, then it
turns out that the blank line is within a `<pre>` block, which means the
paragraph should end at the third line; and, if we consider the
paragraph to end at the third line, then it turns out that there is no
`<pre>` block so the blank line in the middle shouldn't have been
allowed.
Disallow unescaped backticks within any span-level construct other than a code span

Span-level elements that are affected by this are links (linked content,
link ref, url, title), images (alt text, ref, url, title) and
auto-links.

Within URLs, backticks are expected to be escaped to "&#96"; at other
places, backticks are expected to be escaped to "\`".
Syntax guide: Mention that backticks in link text/url/title should be escaped
Merge pull request #3 from roop/find_end_of_para_aware_of_code_spans

Finding the end of a paragraph should be done aware of code spans

Fixes vfmd/vfmd-spec#1