Backtick inline code

This supports `code` with backticks in ddoc. It passes it through the same code that handles the --- example --- sections, so we get exactly the same escaping and highlighting behavior, (and cross referencing when that gets added).

The only difference is it outputs it in a D_INLINECODE macro instead of D_CODE. My default definition of this macro uses html pre with inline style, so it displays as expected.

The test file I used to eyeball it is here:
http://arsdnet.net/dcode/example.d
http://arsdnet.net/dcode/example.html


The backtick syntax is used on Stack Overflow and GitHub so a lot of people are used to it and it looks pretty decent. Raw string literals in the raw text will change due to this, but they typically are inside an example block anyway which continues to work the same as before.

This should be a win for the Markdown advocating crowd with virtually zero cost to everyone else. It also gives me the xml escaping I've been begging for by just backticking it, solving my biggest problem with ddoc!