Allow more permissive globs

[ruipserra]

Hi everyone,

Currently, the README states the following:

Any valid ruby glob may be used.

In sass-rails 4.x this used to be the case. For example, we could structure our project like this:

/* application.scss */

@import "components/**/base.scss";

@media #{$small-mq} {
  @import "components/**/small.scss";
}

@media#{$medium-mq} {
  @import "components/**/medium.scss";
}

@media #{$large-mq} {
  @import "components/**/large.scss";
}

This organization let us manage our scss components quite easily and logically. Cascade order is explicitly stated, and creating a new component is just a matter of following the convention, eg. creating a file component/foo/base.scss.

However, in version 5 (actually since #284), glob syntax is limited to patterns ending in /* or /**/*. This PR provides additional context for the change.

While I understand that supporting any allowed ruby glob pattern may be tricky due to dependency tracking, it would be quite useful if the importer accepted more permissive glob patterns, such as the example above.

On the other hand, the README should clearly document glob restrictions.

Would love to get feedback on this. Thank you.