Include changes from other PRs, resolving conflicts

[rnorth]

Indent the ellipsis between non-adjacent lines (#15)

Use the last line indent so that the result can be
un-indented by the caller. It prevents results like this:

    /* block 1 */
...
    /* block 2 */

turning them into

    /* block 1 */
    ...
    /* block 2 */

which will be dedented by the caller.

Allow selectors on next line (#10)

Allows targeting expressions to be on the next line:

<!--codeinclude-->
<!-- On the same line -->
[Title](long/long/Path) targeting_expr1

<!-- On the next line -->
[Title2](long/long/Path)
targeting_expr2
<!--/codeinclude-->

I kept the usage of a regular expression to find the include definitions, but had to change the implementation to use re.findall and a multiline expression instead of re.match over a single line.

The processing is now separated into three stages:

1. Code-include block extraction.
2. Computing substitutes (patches) for each block.
3. Application of patches to the original markdown.

Resolves #9

Fix inside_block inclusion with multiple curly brackets on the same line (#14)

Add support for any language (#5)

Auto-detect the source language and the alias to use using the Pygments API.

Allow empty titles (#6)

Allow empty title for code snippets. It may be useful for short code fragments.