Improve block level tags detection
Enable HTML parsing inside block level tags

This will allow to grey out tags and make the content explicit (with black color).
Grey out only tag parts in xml/html tags, not the whole content

Extra commas are there for a workaround of ST parsing bug. Without them, ST can't parse multiline scope list correctly.
Allow XML/HTML tags inside list paragraphs
Remove fenced code block definition

This feature will be moved to a new syntax file spesific to GFM.
Change UUIDs of the syntax files

They were the same with the native Markdown plugin's UUDs
Show Setext style headers in the symbol list

Symbol list is a quick TOC of the markdown document. Sublime Text syntax definition files doesn't allow to parse setext syle headers (`====` or `-----`). This makes symbol list non-reliable.

This commit makes a workaround. Instead of showing nothing about setext style headers in the symbol list, it shows `====` or `------` part of it. `-----` is displayed indented like `## ` header.
Fork `Markdown.tmLanguage` file for the GFM version
Create some Markdown files for testing
Implement fenced code blocks with inner syntax highlighting

Credit goes to https://github.com/aziz/knockdown. Code is improved a bit.
Changes and refactoring in default settings
Add menu command for "Markdown GFM"
Update README
Define background color for inline code
Rename syntax files

`Markdown.tmLanguage` -> `Markdown (Standard).tmLanguage`
`Markdown GFM.tmLanguage` -> `Markdown.tmLanguage`

Diff looks terrible, don't mind it.
Regulation of file extension assignments
Update menu
Fork "bold" definition for implementing "strikethrough"

I guess they are too similar.
Implement GFM style strikethrough ("~~")

Shorcut is changed, too. So, when a text is selected and <kbd>~</kbd> is pressed, text will be surrounded with `~~` instead of `<del>` tag.
Update README