Keep track of authors across local commits and branch commits for suggestions

Previously, we would only show the authors based on local commits, but sometimes you want to set a commit author
to that of a commit on another branch. Now, so long as you've viewed the branch's commits, the author will appear
as a suggestion.

pkg/commands/models/author.go

@@ -0,0 +1,13 @@
+package models
+
+import "fmt"
+
+// A commit author
+type Author struct {
+	Name  string
+	Email string
+}
+
+func (self *Author) Combined() string {
+	return fmt.Sprintf("%s <%s>", self.Name, self.Email)
+}