Markdown: multiple improvements and fixes
Improve & fix detection of bold/italic text
In bold and italic text highlighting, underscores and asterics aren't allowed inside, for example **some_text** isn't highlighted correctly. I have improved regex to detect bold and italic text: now asterisks, underscores and escapes are allowed inside.
Improve fenced code blocks
The way to write fenced code blocks varies depending on the Markdown/MultiMarkdown implementation. Some support only 3 backticks, others more than 3 backticks, others both; and others 3 tildes (~). 
Therefore, I think it's a good idea to support all implementations: code blocks can be written with 3 backticks or more, or with 3 tildes or more . Other editors, such as Visutal Studio Code, Atom and Sublime Text highlight fenced code blocks in this way.
Unfortunately, this is only possible using dynamic rules with several RegExpr, which isn't very optimal. I have no problem modifying that if you don't like it.
- Add folding in fenced code blocks.
- Add more keywords to identify languages.
Improve code blocks
- Highlight single code with more than one backticks. 
- Fix: highlight indented code only after an empty line.
Improve highlighting of links and references
Improve metadata highlighting
Previously, some metadata Keys were highlighted anywhere in the document. Now the metadata is highlighted only on the first line of the Markdown document.  
Improves list detection
Through dynamic rules, the indentation of lists is captured, in order to correctly highlight the indented content in them.
Now the text within the lists is highlighted using "dsNormal", so as not to saturate Markdown documents with many colors.
Add support of inline HTML
IncludeRules is used to highlight only HTML tags. 
- Highlight checkboxs in lists.
- Add escape characters .
- Add ##Alerts and ##Modelines in the comments.
- Some minor improvements, such as replacing some RegExpr rules and adding column="0" in some rules.
-  Fenced Markdown code blocks: https://meta.stackexchange.com/questions/125148/implement-style-fenced-markdown-code-blocks/143705#143705
-  Markdown Syntax Documentation. Code: https://daringfireball.net/projects/markdown/syntax#code
-  GitHut - Programming Languages and GitHub: https://githut.info/
-  Most Popular and Influential Programming Languages of 2018: https://stackify.com/popular-programming-languages-2018/
-  Markdown Syntax Documentation. Backslash escapes: https://daringfireball.net/projects/markdown/syntax#backslash
-  MutiMarkdown Metadata: https://fletcher.github.io/MultiMarkdown-5/metadata.html
-  Markdown metadata format: https://stackoverflow.com/questions/44215896/markdown-metadata-format
-  Markdown Syntax Documentation. Inline HTML: https://daringfireball.net/projects/markdown/syntax#html
Reviewers: cullmann, dhaumann, Framework: Syntax Highlighting
Reviewed By: dhaumann, Framework: Syntax Highlighting
Subscribers: kwrite-devel, kde-frameworks-devel
Differential Revision: https://phabricator.kde.org/D23371