Update file parsing to handle lines with only whitespace.

This commit updates file parsing so that lines that contain solely
whitespace (as defined by by Unicode) are ignored. It's fairly common
for people to accidentally create CODEOWNERS files with lines containing
solely whitespace, and as it's semantically meaningless, it's better to
ignore them then raise an error.

Author: captainreality

example_test.go

@@ -8,7 +8,9 @@ import (
 )
 
 func Example() {
-	f := bytes.NewBufferString("src/**/*.c @acme/c-developers")
+	f := bytes.NewBufferString(`src/**/*.c @acme/c-developers
+# The following line should be ignored; it contains only spaces and tabs` +
+		" \t\nsrc/**/*.go @acme/go-developers")
 	ruleset, err := codeowners.ParseFile(f)
 	if err != nil {
 		panic(err)
@@ -19,9 +21,13 @@ func Example() {
 
 	match, err = ruleset.Match("src/foo.rs")
 	fmt.Println(match)
+
+	match, err = ruleset.Match("src/go/bar/bar.go")
+	fmt.Println(match.Owners)
 	// Output:
 	// [@acme/c-developers]
 	// <nil>
+	// [@acme/go-developers]
 }
 
 func ExampleParseFile() {

parse.go

@@ -30,7 +30,7 @@ func ParseFile(f io.Reader) (Ruleset, error) {
 		line := scanner.Text()
 
 		// Ignore blank lines and comments
-		if len(line) == 0 || line[0] == '#' {
+		if len(strings.TrimSpace(line)) == 0 || line[0] == '#' {
 			continue
 		}