Ability to set user environment path (bin folder) for users to use their own kernels

### Proposed change
I want to have users with custom username and password, which is a feature already achieved by this repo.
However, all users can only use the kernel as same as the one which launched jupyterhub instance, if they want to run with specific version libraries, they will have to ask admin to do that, even they already created such env in same machine.

If we launch jupyterhub without this plugin, and config as following:
```
c.LocalProcessSpawner.shell_cmd = ["bash", "-l", "-c"]
```

Then for users, they can set `PATH` in `~/.profile` to specify their own conda location:
```
export PATH="$HOME/miniconda/bin:$PATH"
```
Then they can use jupyterhub with their own envs.

Or, might we can bind local users to jupyterhub users, then can just do samething?