Add option to force TextFormatter to use colors

I'm using Logrus with Revel, which does some fancy stuff for autoreloading code in dev. A parent process watches for changes, and recompiles/forks a child that isn't attached directly to a TTY. However, I'd still like to see pretty colours in this case, since the output still ends up on a TTY.

With this change, you can do something like

```go
log.Formatter = &logrus.TextFormatter{ForceColors: true}
```

to bypass checking for a TTY.