Add Support for Wiki Images

[vsch]

New Extension overview for flexmark-ext-wikilink

flexmark-java extension for wiki links

Converts references that are wrapped in [[]] into wiki links with optional text separated by
|.

Will also convert ![[]] to image links if IMAGE_LINKS extension option is enabled.

Options:

• DISABLE_RENDERING default false, if true then rendering of wiki links is disabled and they
  will render as plain text of the element node

• IMAGE_PREFIX default "", prefix to add to wiki link page reference

• IMAGE_LINKS default false, true will enable ![[]] image link syntax

• IMAGE_FILE_EXTENSION default "", extension to be added to wiki image file refs

• LINK_FIRST_SYNTAX default false, if true then [[page ref|link text]] syntax is used,
  otherwise [[link text|page ref]] syntax. Affects both link and image wiki references.

• LINK_PREFIX default "", prefix to add to wiki link page reference

• LINK_FILE_EXTENSION default "", extension to be added to wiki link page refs

[vsch]

@vmassol, wiki extension now supports wiki images as an option.

[vmassol]

Tested and working!