support extension `escaped_line_breaks`

[Timothee Cour]

full context: https://github.com/nim-lang/Nim/issues/9291#issuecomment-432351178
it's useful for editors that automatically trim trailing spaces

[Ju]

Will support it in the next version.

[Oscar N.]

Note that this is part of the GFM spec

[Ju]

v0.3.4 released. Nim-markdown now supports hard line breaks by both \ and trailing whitespaces.

test "inline break":
  check markdown("hello\nworld") == "<p>hello\nworld</p>"
  check markdown("hello\\\nworld") == "<p>hello<br />\nworld</p>"
  check markdown("hello  \nworld") == "<p>hello<br />\nworld</p>"