Added 'Get started quickly' section
Remove a compiler warning

In 7b48ac95, the bufcstr prototype has been changed. This lead to a
warning since bufgrow is meant to change the allocated size to the given
buffer but the latter is a const.
Add a note about callback overriding [ci skip]

To avoid unexpected outputs, the callback should return a HTML element
with a level that matches the level of the overwritten callback.

Refs #336
Strip image links in StripDown renderer
Merge pull request #356 from MrZYX/stripdown_images

Let StripDown handle image tags
Merge pull request #355 from vmg/buffer-warning

Remove a compiler warning
Merge pull request #353 from evolve2k/patch-1

Added 'Get started quickly' section to README [ci skip]
Follow up to #353 [ci skip]

Improve a bit the README and use 1.9 hash syntax in code examples
Change MKDEXT_NO_INTRA_EMPHASIS to not start after alphanumerics

No behavior should change if the NO_INTRA_EMPHASIS flag is not set.
With it
set, the following behavior changes should be seen:  (The parentheses
in the
following examples can be replaced by any non-alphanumeric character.)

- "(_italics_)" is italic
- "this(_is_)weird" italicizes the "is"

We were already checking for non-alphanumerics  on the closing boundary;
that is, "_this_()" would have rendered as italicized even though the
closing
underscore wasn't followed by a space. This commit just brings the same
behavior to the opening underscore.
Update README.markdown
Merge pull request #362 from nemesis/patch-1

Update README.markdown [ci skip]
Merge pull request #361 from bigjason/intra-emph-refine

Change MKDEXT_NO_INTRA_EMPHASIS to not start after alphanumerics
Add a changelog entry for #361 [ci skip]
Avoid adding extra conditions

If `isalnum` returns true for the given char, the latter will be
different from `>` or ')' anyway. The introducing commit is e153686c.
Fix typo
Merge pull request #364 from mrueg/master

Fix typo
Merge pull request #363 from vmg/follow-up-361

Follow up to #361
Introduce _isalnum checking ASCII alphanumerics

`_isalnum` checks if the given character is an alphanumeric character
in the ASCII range. (The code is borrowed from github-markdow.)

This commit enables emphasis inside of sentences in multi-byte languages
when the the NO_INTRA_EMPHASIS flag is set.

For example, given the following sentence (in Chinese):

    中文內無空白的**粗體**也應加粗。

The two characters surrounded by asterisks should be bold.

This frees multi-byte language users from adding extra spaces into
sentences and is exactly how markdown on Github behaves.
Merge pull request #366 from Domon/intra-emph-multibyte

Enable emphasis in multi-byte sentences when no_intra_emphasis is set
Remove the testing part from the README [ci skip]

This is already explained in the contribution guideline