Add data-dir and extensions-dir flag

[foresthoffman]

It would make sense to also have --extensions-dir and --user-data-dir flags, since both of those are available on code-server.

https://github.com/codercom/code-server/blob/5f40ebb8457380aa1e9955302d9e41a6b042ef14/packages/server/src/cli.ts#L23-L24

[ammario]

Related #29

I think it's best to have these as environment variables.

[SscSPs]

I think env variables are good, but config file would be better, won't have to pass them everytime..
We can have both, env variables having more priority than config. And config having more priority than defaults

[foresthoffman]

Environment variables can be easily placed in a user's .bashrc or .bash_profile.

[SscSPs]

My reason for having a different config file is that other apps should not accidently use those variables. I know this is rare, but possible.
Noone will be going in some other application's configs accidently

[ammario]

It's not worth worrying about the chance of another application using an environment variable as specific as VSCODE_SETTINGS_DIR