feat(pagination): implement paginated output

Implement a `pager` config option. If configured, `cheat` will
automatically pipe output through the configured pager (where
appropriate).

internal/display/display.go

@@ -0,0 +1,37 @@
+package display
+
+import (
+	"fmt"
+	"os"
+	"os/exec"
+	"strings"
+
+	"github.com/cheat/cheat/internal/config"
+)
+
+// Display writes output either directly to stdout, or through a pager,
+// depending upon configuration.
+func Display(out string, conf config.Config) {
+	// if no pager was configured, print the output to stdout and exit
+	if conf.Pager == "" {
+		fmt.Print(out)
+		os.Exit(0)
+	}
+
+	// otherwise, pipe output through the pager
+	parts := strings.Split(conf.Pager, " ")
+	pager := parts[0]
+	args := parts[1:]
+
+	// run the pager
+	cmd := exec.Command(pager, args...)
+	cmd.Stdin = strings.NewReader(out)
+	cmd.Stdout = os.Stdout
+
+	// handle errors
+	err := cmd.Run()
+	if err != nil {
+		fmt.Fprintln(os.Stderr, fmt.Sprintf("failed to write to pager: %v", err))
+		os.Exit(1)
+	}
+}