Fix handling of comments in import groups (#11)

I noticed #10 introduced a regression in that a comments sitting on
a separate line are now ignored - leading to the `importInfo` "value"
being set to an empty string. This trips things up further along as
empty string implies a separate group. I've tried to address this
by retaining "value" as the raw value (renamed to `lineValue` for
clarity), and adding a new field to `importInfo` which contains the
import path, if valid for the given line.

Use go/scanner to extract import path (#10)

* Use go/scanner to extract import path

This should be more reliable than stripping out unwanted parts of
the import line. Most notably it resolves #9

Note that I've not been able to verify this on an exhaustive set of
examples. Also note that there are still potential gotchas around
mutli line comments.. debatable whether or not those are worth
addressing (and definitely not wanting to cover in this PR, given
it has immediate benefits).

* Improve error line; ensure a string is printed

Ability to optionally ignore generated files (#8)

This gives a user the ability to optionally ignore files generated by the `go generate` command. 

Fixes #7