Allow imports from node modules with default install

[ghost]

I ran into an issue when trialing the new gem, whereby @import statements wouldn't load common Sass libraries without specifying the full path to the node_modules directory.

This allows simple imports such as @import 'glidecss/base'; to work with the default install.

[AndreyAzimov]

Yeah, I managed to fix it only using absolute path like: @import '../../../node_modules/bootstrap/scss/bootstrap'

[ghost]

Exactly what I was doing before the change. The trouble is, if a library has dependencies, they won't get resolved. For example, https://github.com/thelucid/glidecss has a dependency on modern-normalize, so that also exists in the node_modules directory and needs to be in the load path.