Add support for multi-line comments

[MatthewDolan]

When running this formatter against our code, we noticed that it was removing all but the last line from multi-line block comments on imports.

It looks like this is referenced as a TODO in the README ("Support multiple lines of comment in import block"), but just wanted to call it out as an explicit issue / feature request.

Example

For example, for a file like this:

package example

import (
	// A block comment about why this package is being imported.
	//
	// More information about why this package is being imported.
	_ "github.com/example/nested"
)
...

Expected output

We expected nothing to happen to this file. The multi-line comment block would be treated as associated with the import below it and kept together.

Actual output

The formatter removes all but the last line of the comment.

> gci -d .
diff -u example.go.orig example.go
--- example.go.orig	2020-09-01 08:10:37.000000000 -0700
+++ example.go	2020-09-01 08:10:37.000000000 -0700
@@ -1,8 +1,7 @@
 package example
 
 import (
-	// A block comment about why this package is being imported.
-	//
+
 	// More information about why this package is being imported.
 	_ "github.com/example/nested"
 )

[daixiang0]

It may be hard to implement only with string detect, AST may help it.

[daixiang0]

After #50, GCI would not format those :)

Feel free to reopen it if still exist.