Add configurable flake8 executable

[ruhulio]

The context for this change:

• I'm using virtural envs with different versions of Python so each of them has their own install of pyls and dependencies.

• I'm using Sublime Text with Sublime LSP configured to use the pyls from the project's virtual env.

  Example configuration

  ```json { "settings": { "LSP": { "pyls": { "enabled": true, "command": [ "/virtualenvs/important-project/bin/pyls" ], "settings": { "pyls": { "plugins": { "flake8": { "enabled": true, "config": "/work/code/important-project/.flake8" } } } } } } } } ```

Within that context, the flake8_lint plugin tries to start flake8 via a process; either directly or as a module. In both cases, this happens outside of the virtual env so it does not pick up the correct flake8 executable or module.

This change allows for adding an optional explicit executable configuration for flake8 that can point directly to the virtual env's installed version.

This change should be backwards compatible as it defaults to the previous behavior when there is no executable provided in the configuration.

The other plugin's do not have a similar issue as they do not use a separate process.

[palantirtech]

Thanks for your interest in palantir/python-language-server, @ruhulio! Before we can accept your pull request, you need to sign our contributor license agreement - just visit https://cla.palantir.com/ and follow the instructions. Once you sign, I'll automatically update this pull request.