Handle inline code inside bold/italics/links

[ariabuckles]

Currently, _italic text and code with_ pointers* doesn't match correctly. [Note: apparently it doesn't match correctly in gfm, either, so that's some validation :/.]

Handling this correctly (especially with simple-markdown's regex-based design) is tricky, but important.

We might have to write a custom matcher function for the rules where this is most important, instead of using regex matchers. For an example, see the math matching in Perseus.

[kevinbarabash]

@ariabuckles would *code example: console.log(2 * 2);* be a failing example?