cli: don't always use color (#233)

Some benefits:
- A user who writes `configlet lint > output.txt` will no longer see
  color codes in that file.
- It becomes cleaner to write integration tests and regression tests
  that involve `configlet lint`.

src/cli.nim

@@ -185,8 +185,21 @@ proc showVersion =
   echo &"{configletVersion}"
   quit(0)
 
+proc shouldUseColor(f: File): bool =
+  ## Returns true if we should write to `f` with color.
+  existsEnv("CI") or
+    (isatty(f) and not existsEnv("NO_COLOR") and getEnv("TERM") != "dumb")
+
+let
+  colorStdout* = shouldUseColor(stdout)
+  colorStderr* = shouldUseColor(stderr)
+
 proc showError*(s: string) =
-  stderr.styledWrite(fgRed, "Error: ")
+  const errorPrefix = "Error: "
+  if colorStderr:
+    stderr.styledWrite(fgRed, errorPrefix)
+  else:
+    stderr.write(errorPrefix)
   stderr.write(s)
   stderr.write("\n\n")
   showHelp(exitCode = 1)

src/helpers.nim

@@ -1,4 +1,5 @@
 import std/[algorithm, os, terminal]
+import "."/cli
 
 template withDir*(dir: string; body: untyped): untyped =
   ## Changes the current directory to `dir` temporarily.
@@ -21,6 +22,10 @@ proc setFalseAndPrint*(b: var bool; description: string; details: string) =
   ## Sets `b` to `false` and writes a message to stdout containing `description`
   ## and `details`.
   b = false
-  stdout.styledWriteLine(fgRed, description & ":")
+  let descriptionPrefix = description & ":"
+  if colorStdout:
+    stdout.styledWriteLine(fgRed, descriptionPrefix)
+  else:
+    stdout.writeLine(descriptionPrefix)
   stdout.writeLine(details)
   stdout.write "\n"