Add a prerelease method to version

This method returns the joined components of the prerelease field. It's
intended use is really to see if there is a prerelease or not, so a bool could
also work, possibly.

version.go

@@ -144,3 +144,16 @@ func (v *buildlessVersion) comparePre(other *Version) int {
 
 	// build is not used for semver comparison.
 }
+
+// Prerelease returns the prerelease portion of a Version. If the version had
+// no prerelease or is nil, the empty string is returned.
+//
+// Note: This method is considered experimental, and may be removed or changed
+// before v1.0.0
+func (v *Version) Prerelease() string {
+	if v == nil {
+		return ""
+	}
+
+	return strings.Join(v.pre, ".")
+}

version_test.go

@@ -181,3 +181,29 @@ func TestCompare_nil(t *testing.T) {
 		t.Errorf("two nil versions are not equal")
 	}
 }
+
+func TestVersion_Prerelease(t *testing.T) {
+	mustParse := func(s string) *semver.Version {
+		v, _ := semver.Parse(s)
+		return v
+	}
+
+	tcs := map[string]struct {
+		in  *semver.Version
+		out string
+	}{
+		"no prerelease":         {mustParse("1.0.0"), ""},
+		"prerelease":            {mustParse("1.0.0-alpha"), "alpha"},
+		"multi-part prerelease": {mustParse("1.0.0-rc.0"), "rc.0"},
+		"nil version":           {nil, ""},
+	}
+
+	for name, tc := range tcs {
+		t.Run(name, func(t *testing.T) {
+			out := tc.in.Prerelease()
+			if out != tc.out {
+				t.Error("bad prerelease. got:", out, "wanted:", tc.out)
+			}
+		})
+	}
+}