Augmented paragraph movement commands

Move by paragraph by default, but by block with C-u prefix.

The addition of the separate keybindings M-[ and M-] for movement by
block was problematic because the escape sequences sent for the delete
key by some keyboards begin with M-[.  When running in the terminal,
this causes Emacs to trigger the block movement command and insert a
control character (corresponding to the remainder of the sequence).

To remedy this situation, the specific M-[ and M-] block movement
keybindings have been removed.  Instead, the usual M-{ and M-} paragraph
movement commands have been augmented: skip by paragraphs, as defined,
or by block when the C-u prefix is given.

Some relevant Emacs history:

> M-[ was an Emacs 18 binding which was moved to M-{ in Emacs 19,
> precisely because it conflicted with the escape sequences produced by
> some common keyboards.

Source: <http://www.nongnu.org/emacs-tiny-tools/keybindings/index-body.html>
Fix markdown-font-lock-extend-region return value

Don't change font-lock-beg if re-search-forward returns nil.
Furthermore, markdown-font-lock-extend-region should return nil if it
does not modify font-lock-beg and font-lock-end.
Fix repeated calls to extend region function

markdown-font-lock-extend-region should return nil when the region has
been extended to contain the current block.  It appears that this was
not happening previously, rather, the function kept extending the region
to contain more and more blocks.
Fix arguments for block movement functions

Functions markdown-[beginning|end]-of-block have zero arguments.