add an option to filter file content for preview

Mentioned in #5.

comp.go

@@ -20,6 +20,7 @@ var (
 		"scrolloff",
 		"tabstop",
 		"ifs",
+		"previewer",
 		"shell",
 		"showinfo",
 		"sortby",

eval.go

@@ -62,6 +62,8 @@ func (e *SetExpr) eval(app *App, args []string) {
 		gOpts.tabstop = n
 	case "ifs":
 		gOpts.ifs = e.val
+	case "previewer":
+		gOpts.previewer = e.val
 	case "shell":
 		gOpts.shell = e.val
 	case "showinfo":

opts.go

@@ -6,6 +6,7 @@ type Opts struct {
 	scrolloff int
 	tabstop   int
 	ifs       string
+	previewer string
 	shell     string
 	showinfo  string
 	sortby    string
@@ -21,7 +22,6 @@ func init() {
 	gOpts.preview = true
 	gOpts.scrolloff = 0
 	gOpts.tabstop = 8
-	gOpts.ifs = ""
 	gOpts.shell = envShell
 	gOpts.showinfo = "none"
 	gOpts.sortby = "name"

ui.go

@@ -6,6 +6,7 @@ import (
 	"fmt"
 	"log"
 	"os"
+	"os/exec"
 	"path"
 	"sort"
 	"strconv"
@@ -255,9 +256,19 @@ func (win *Win) printr(reg *os.File) error {
 		return fmt.Errorf("printing regular file: %s", buf.Err())
 	}
 
-	reg.Seek(0, 0)
+	if len(gOpts.previewer) != 0 {
+		cmd := exec.Command(gOpts.previewer, reg.Name())
 
-	buf = bufio.NewScanner(reg)
+		out, err := cmd.Output()
+		if err != nil {
+			log.Printf("previewing file: %s", err)
+		}
+
+		buf = bufio.NewScanner(bytes.NewReader(out))
+	} else {
+		reg.Seek(0, 0)
+		buf = bufio.NewScanner(reg)
+	}
 
 	for i := 0; i < win.h && buf.Scan(); i++ {
 		win.print(2, i, fg, bg, buf.Text())