Preformatted inline code

[martincizek]

Add support for interpreting <code> as a preformatted inline element and rendering Markdown accordingly. Fix #318.

This PR builds on previous PRs #315 and #317.

Quality assurance:

• Changes in collapse-whitespace.js also submitted to the original collapse-whitespace project. The code is the very same except for coding style nuances.
• All previous tests in the collapse-whitespace project pass. New tests added for the new behavior, checking it matches browser behavior.
• The rest of Turndown is unaffected without preformattedCode enabled.
• There is not much complexity added, actually preformatted code behaves almost like block elements.
• We believe it is a core thing related to basic HTML semantics, just like <pre> for block preformatted code. So merging this to Turndown's core is justifiable.

Performance:

• Couple of lines added.
• No new costly operations.

[martincizek]

Rebased to the new master and removed a JS test case which is actually covered in HTML tests.

[pavelhoral]

LGTM

[martincizek]

LGTM

Thanks for your "interrogation" in chat. :-)

I also checked that nothing has changed in the standard code rule. And it's OK, the code rule still clearly mirrors the CommonMark spec, so it will work correctly both with collapsed and preformatted content.