Handle the --color flag via proper global state

Use `color.NoColor` as a global progressively modified as one descends
down command flags. Allows non-surprising operation when --color is
accepted at multiple layers, and even makes this behave predictably:

    lotus-miner --color actor control list --color=false

cli/util.go

@@ -3,10 +3,13 @@ package cli
 import (
 	"context"
 	"fmt"
+	"os"
 	"time"
 
+	"github.com/fatih/color"
 	"github.com/hako/durafmt"
 	"github.com/ipfs/go-cid"
+	"github.com/mattn/go-isatty"
 
 	"github.com/filecoin-project/go-state-types/abi"
 
@@ -15,6 +18,13 @@ import (
 	"github.com/filecoin-project/lotus/chain/types"
 )
 
+// Set the global default, to be overridden by individual cli flags in order
+func init() {
+	color.NoColor = os.Getenv("GOLOG_LOG_FMT") != "color" &&
+		!isatty.IsTerminal(os.Stdout.Fd()) &&
+		!isatty.IsCygwinTerminal(os.Stdout.Fd())
+}
+
 func parseTipSet(ctx context.Context, api v0api.FullNode, vals []string) (*types.TipSet, error) {
 	var headers []*types.BlockHeader
 	for _, c := range vals {