Add support for customisable TODO keywords

[alex0]

@FlorianF I have proposed a syntax for inline properties here sometime ago:

Syntax for inline properties::

From the discussion here I think we have a cool proposal. We want this syntax: - [[title::The Picture of Dorian Gray]] is a [[type::book]] written by [[author::Oscar Wilde]]. as a more compact way to write this block: - type:: book title:: The Picture of Dorian Gray author:: Oscar Wilde [[The Picture of Dorian Gray]] is a [[book]] written by [[Oscar Wilde]]. Rendered, with links, as: The Picture of Dorian Gray is a book written by Oscar Wilde. So it’s rendered just like: - [[Th…

CC @futurized in case you are interested