Prevent error "Cannot read property '0' of null" when there is no file

Ref https://github.com/segmentio/myth/issues/62
Merge pull request #4 from MoOx/patch-1

Prevent error "Cannot read property '0' of null" when there is no file
Recursive @import

Fix #5
Merge pull request #6 from MoOx/patch-2

Recursive @import
Minor improvements and tests
0.2.0