Import some settings from the native Markdown plugin

When using this plugin, it's better to disable the native Markdown plugin. However, that plugin has 2 simple but important `.tmPreferences` files. This commit imports them.
Improve the symbol list definition

The list that triggers with <kbd>Ctrl+R</kbd>
Add dark theme
Mark the text between "`" chars with a different scope

This makes it possible to press Ctrl+Shift+Space in an inline codeblock and get the inner code selected without the "`" characters.
Merge pull request #85 from maliayas/fixes

Fixes
Merge pull request #87 from maliayas/inline-codeblock-scope

Mark the inner part of inline code blocks with a different scope
Merge pull request #84 from maliayas/dark-theme

Add dark theme