feat(pagination): implement paginated output

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

internal/config/config.go

@@ -5,6 +5,7 @@ import (
 	"io/ioutil"
 	"os"
 	"path/filepath"
+	"strings"
 
 	cp "github.com/cheat/cheat/internal/cheatpath"
 
@@ -19,6 +20,7 @@ type Config struct {
 	Cheatpaths []cp.Cheatpath `yaml:"cheatpaths"`
 	Style      string         `yaml:"style"`
 	Formatter  string         `yaml:"formatter"`
+	Pager      string         `yaml:"pager"`
 }
 
 // New returns a new Config struct
@@ -111,5 +113,10 @@ func New(opts map[string]interface{}, confPath string, resolve bool) (Config, er
 		conf.Formatter = "terminal16m"
 	}
 
+	// if a pager was not provided, set a default
+	if strings.TrimSpace(conf.Pager) == "" {
+		conf.Pager = ""
+	}
+
 	return conf, nil
 }

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)
+	}
+}