Set code block language when inserting, add auto-completion and auto-indentation to codeblocks

the way to insert a codeblock need to be improved

• now, if I want to insert a block, i need to print “/code” and then press Enter and then select programming languages such as C++ by mouse, that’s too cumbersome!

• so ,I hope I can just print ```cpp``` and then press Enter to create a C++ codeblock, maybe it is conflicted with quote function, but I think Quote should be ‘quoted content’ rather than ``quoted content`` or ``quoted content + right arrow

Auto-completion(such as {}() ) and auto-indentation are not supported now, which should be supported.

The code highlighting doesn't work when my cursor is on the code block, I'm not sure if this is because I'm using custom css