add support for reading template from stdin

[deitch]

Fixes #1247

Some comments:

• if --tty is not provided and the template is read from stdin, then it defaults to --tty=false
• if --tty=true as a CLI flag and the template is read from stdin, return an error
• if --name as a CLI flag is not provided and the template is read from stdin, return an error
• uses - as the indicator for stdin, inline with what most Unix-style programs do

[AkihiroSuda]

Thanks