implement code -r (reuse window)

[shi-yan]

vscode has this command line

code -r filename

This command will open the file with an existing vscode window.

Having this command is convenient, because I can open any file from vscode's built-in terminal.

I wish code-server has something similar.

basically, within the code-server's built-in terminal, if I type code-server -r filename, the file will be opened by code-server in the same browser window.

[avelino]

Duplicate: #164