Allow starting `purs ide server` in alternative path

[ChrisCoffey]

On many of the purescript projects I work on we have the purs version pinned in the package.json. That often creates compatibility issues between the project versions & my globally installed versions. This change enables prepending an optional "runner" command in front of the purs ide server & purs ide client commands.

I also added documentation for the settings (happy to split that out into a separate PR if you prefer.

[ChrisCoffey]

Thanks for the review @chexxor.

My specific use case was executing purs via another "runner" program. Your suggestion to allow an explicit path to purs is a great generalization. The most recent commit now allows you to run let g:psc_ide_server_runner=['yarn', 'purs'] if you want to execute purs via yarn, or let g:psc_ide_server_runner=['/path/to/my/purs'] to execute from an absolute path.

[ChrisCoffey]

Thanks for the review @coot . I like the idea of buffer local runner & port variables!

[ChrisCoffey]

@coot would you mind taking another look?

[chexxor]

@ChrisCoffey Mind if I ask why you closed this? Seems like this project needs more maintainers to help manage PRs like this.