Allow custom implementations of InlineParsers

[lalunamel]

Hi there!

Continuing with the theme of extensibility I'd like to be able to specify the inline parser used to parse markdown.

In my particular use case I'd like to disable a few features of markdown (e.g., inline images) and add a few of my own (e.g., @name will be recognized and parsed as something specific).

Proposal:
Add a method to Builder, public Builder inlineParser(InlineParser parser) that allows passing an implementation of InlineParser to Parser.

e.g.,

Parser.builder().inlineParser(new AtMentionParser()).build()

[lalunamel]

#69

[robinst]

Released in 0.9.0 with some other changes: https://github.com/atlassian/commonmark-java/releases/tag/commonmark-parent-0.9.0