Add highlighting for some thirdparty plugins

Adds syntax highlighting for https://github.com/facelessuser/BracketHighlighter/ and https://github.com/SublimeText/WordHighlight. It doesn't affect users who don't have these plugins.
Add link reference definitions to the symbol list

Maybe it should be better in this way: http://www.macdrifter.com/2012/08/making-a-sublime-text-plugin-markdown-reference-viewer.html
Update README
Revert "Hide themes from `Preferences > Color scheme` menu"

This reverts commit e3cb36a1950e766a8d105c4fbb9065e6b183d164.

It causes a bug that when theme is updated, ST doesn't reload it.
Code refactoring in Markdown.tmLanguage
Highlight inside keyboard shortcut tags (<kbd>)

... like GitHub does.
Improve strikethrough
Add option to keep current line centered

Option names are prefixed with `me_`, abbr of "Markdown Editing".
Improve fenced code blocks
Allow HTML/XML in blockquote
Make blockquotes lighter
Inside blockquotes, lighten some stuff, too

So everything is a tone lighter. Colors are calculated mathematically.
Make the "space after `>`" optional for blockquotes

It is optional in GFM.
Update README
Improve bold and italic keybindings

Using Ctrl+B and Ctrl+I directly is not a bad idea if restricted only to markdown. It makes things simpler. I can't see any side effect.

OTOH, when a user makes a text bold, the next thing he will do is most of the time moving the cursor to the right of the bold text. This commit implements this. So there is no need of extra key strokes like `right arrow` or `end` keys. We need to keep the user focused on the text, not the key presses.

This commit also makes it possible to make a _word_ bold/italic without selecting it. Just move the cursor to the word (at the beginning, in the middle or at the and) and press <kbd>Ctrl</kbd> <kbd>B/I</kbd>. Voila! This makes this scenario possible:

* type a word
* press <kbd>Ctrl</kbd> <kbd>B</kbd> to make it bold
* type space
* type next word

This means no extra key strokes like <kbd>Left</kbd>, <kbd>Right</kbd>, <kbd>End</kbd>, <kbd>Tab</kbd>. Only relevant key strokes.
Disable native Markdown package automatically on plugin install

It causes some conflicts and doesn't have any improvements over MarkdownEditing plugin.
Remove link reference definitions from the symbol list

It was a bad idea :(