Add multiline matches

[sapkra]

Currently it's impossible to match a string with line breaks because convertLineBreaks replaces all of them with <br/> tags which causes the matcher to get get a splitted string.

e.g.
Content:

```foo
bar```

Matcher gets:

```foo

bar```

[milesj]

I wonder if we move convertLineBreaks from running at the start (https://github.com/milesj/interweave/blob/master/packages/core/src/Parser.ts#L270) to running per node after matchers have ran.

Theoretically it should work and be easy, but who knows :P