feat(overrides): Add additional prefix/suffix support

Allows a higher-level prefixes and suffixes by allowing it directly on
`@theme-*` options, which are what are set directly against Tmux's
options.

These are the prefix/suffix options added:

- `@theme-status-left-prefix`
- `@theme-status-left-suffix`
- `@theme-status-right-prefix`
- `@theme-status-right-suffix`
- `@theme-window-status-current-prefix`
- `@theme-window-status-current-suffix`
- `@theme-window-status-prefix`
- `@theme-window-status-suffix`

As they are applied on the highest level, it means they work on ALL
themes, including the `default` theme.

The `@themepack-*` prefix/suffix options still work too, but only for
themes that uses them and have three separate text areas on the left and
right status.

test/setup.go

@@ -0,0 +1,60 @@
+package test
+
+import (
+	"log"
+	"os"
+	"testing"
+
+	"github.com/jimeh/go-tmux"
+	"github.com/stretchr/testify/assert"
+)
+
+var tm = tmux.New()
+var tmuxVersion = os.Getenv("TMUX_VERSION")
+var tmuxBinPath = os.Getenv("TMUX_BIN_PATH")
+var tmuxConfig = os.Getenv("TMUX_CONFIG")
+
+func tmuxSetup() {
+	if tmuxBinPath != "" {
+		tm.BinPath = tmuxBinPath
+	}
+	tm.SocketPath = "./tmux-test-socket"
+
+	if tmuxConfig != "" {
+		tmuxConfig = "./tmux.conf"
+	}
+
+	_, err := tm.Exec("-f", tmuxConfig, "new-session", "-d", "sleep", "30")
+	if err != nil {
+		log.Fatal(err)
+	}
+}
+
+func tmuxTearDown() {
+	_, err := tm.Exec("kill-server")
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	if tm.SocketPath != "" {
+		err = os.Remove(tm.SocketPath)
+		if err != nil {
+			log.Fatal(err)
+		}
+	}
+}
+
+func tmuxHasOptions(t *testing.T, theme string, s tmux.Scope, m tmux.Options) {
+	opts, err := tm.GetOptions(s)
+	assert.NoErrorf(t, err, `%s: Failed to get options`, theme)
+
+	if err == nil {
+		for k, v := range m {
+			_, ok := opts[k]
+			assert.Truef(t, ok, `Key "%s" is not available in %s`, k, theme)
+			if ok {
+				assert.Equalf(t, v, opts[k], `Key "%s" in "%s"`, k, theme)
+			}
+		}
+	}
+}