cmd: add optional silent fail mode on no specific lexer

Makes use of chroma easier in/as less(1) preprocessors.

https://manpages.debian.org/less#INPUT_PREPROCESSOR
  export LESSOPEN='| p() { chroma --fail "$1" || cat "$1"; }; p "%s"'

https://manpages.debian.org/lesspipe#USER_DEFINED_FILTERS
  ln -s ~/go/bin/chroma ~/.lessfilter

Author: scop

Diff for: README.md

@@ -255,6 +255,24 @@ A command-line interface to Chroma is included. It can be installed with:
 go get -u github.com/alecthomas/chroma/cmd/chroma
 ```
 
+The CLI can be used as a preprocessor to colorise output of `less(1)`,
+see documentation for the `LESSOPEN` environment variable.
+
+The `--fail` flag can be used to suppress output and return with exit status
+1 to facilitate falling back to some other preprocessor in case chroma
+does not resolve a specific lexer to use for the given file. For example:
+
+```shell
+export LESSOPEN='| p() { chroma --fail "$1" || cat "$1"; }; p "%s"'
+```
+
+Replace `cat` with your favourite fallback preprocessor.
+
+When invoked as `.lessfilter`, the `--fail` flag is automatically turned
+on under the hood for easy integration with [lesspipe shipping with
+Debian and derivatives](https://manpages.debian.org/lesspipe#USER_DEFINED_FILTERS);
+for that setup the `chroma` executable can be just symlinked to `~/.lessfilter`.
+
 <a id="markdown-whats-missing-compared-to-pygments" name="whats-missing-compared-to-pygments"></a>
 ## What's missing compared to Pygments?
 

Diff for: cmd/chroma/main.go

@@ -7,6 +7,7 @@ import (
 	"io/ioutil"
 	"os"
 	"os/signal"
+	"path"
 	"runtime"
 	"runtime/pprof"
 	"sort"
@@ -43,6 +44,7 @@ command, for Go.
 		Trace      bool             `help:"Trace lexer states as they are traversed."`
 		Check      bool             `help:"Do not format, check for tokenisation errors instead."`
 		Filename   string           `help:"Filename to use for selecting a lexer when reading from stdin."`
+		Fail       bool             `help:"Exit silently with status 1 if no specific lexer was found."`
 
 		Lexer     string `help:"Lexer to use when formatting." default:"autodetect" short:"l" enum:"${lexers}"`
 		Style     string `help:"Style to use for formatting." default:"swapoff" short:"s" enum:"${styles}"`
@@ -105,6 +107,10 @@ func main() {
 		}()
 		defer pprof.StopCPUProfile()
 	}
+	if path.Base(os.Args[0]) == ".lessfilter" {
+		// https://manpages.debian.org/lesspipe#USER_DEFINED_FILTERS
+		cli.Fail = true
+	}
 
 	var out io.Writer = os.Stdout
 	if runtime.GOOS == "windows" && isatty.IsTerminal(os.Stdout.Fd()) {
@@ -238,6 +244,9 @@ func listAll() {
 func lex(ctx *kong.Context, path string, contents string) chroma.Iterator {
 	lexer := selexer(path, contents)
 	if lexer == nil {
+		if cli.Fail {
+			ctx.Exit(1)
+		}
 		lexer = lexers.Fallback
 	}
 	if rel, ok := lexer.(*chroma.RegexLexer); ok {