Option to provide absolute path to pyls

Hi,
Since virtual-environments are very commonly used, its a bit cumbersome to keep installing `pyls` in each environment.
Is there a way to set an absolute path to the `pyls` ( or perhaps I missed this configuration ? )

for eg.
```
➯ ➯ ➯ which pyls
~/anaconda3/bin/pyls
```
vs
```
➯ ➯ ➯ which python
~/anaconda3/envs/my-env/bin/python
```

I'd like to set `pyls` to point to 1st one.

Start by locating the configuration or entry point that selects the pyls executable; the issue does not name a file or test. Verify how the existing Python environment is chosen, then confirm that users can provide an absolute pyls path independent of the active virtual environment.

索引モデルが issue の本文から書いたものです。