Add workspaceRoot and bundler support

[wingrunr21]

This PR introduces support for interpolating the workspaceRoot into the linter path and running the linter via bundler. This should resolve #188 and #187.

The linter path will be resolved in the following order:

1. If a path is set on the linter, that will be used. ${workspaceRoot} will be interpolated.
2. It will attempt to find the linter name via bundler
3. It will call the linter name (eg the linter is installed globally)

This also makes a change to the directory in which the linter is executed. Before it would execute in the project root and fall back to the target file's working directory. Now it does the opposite. This was a requirement to support #188. This has the potential to break people who have set the linter path to be relative to the workspace root. The upgrade path would be to use the variable interpolation instead. See here

• Build pass!
• Follow VS Code Coding Guidelines

[wingrunr21]

ping @rebornix