[Core] Remove color when stdout is not a tty

[jiasli]

Fix #2924: JSONC is not parsed successfully by json.loads in Python2

On Linux, when the output format is set to jsonc or yamlc whether by changing the ~/.azure/config file or by using --output, there will be ANSI control sequences for coloring like ^[[94m in stdout. If stdout is redirected, downstream commands like jq will fail:

$ az vm list --output jsonc | jq '.[0]'
parse error: Invalid numeric literal at line 3, column 6

$ az vm list --output jsonc > out.txt
$ cat out.txt --show-nonprinting
[
  {
    ^[[94m"additionalCapabilities"^[[39;49;00m: ^[[34mnull^[[39;49;00m,
    ^[[94m"availabilitySet"^[[39;49;00m: ^[[34mnull^[[39;49;00m,

This PR detects whether stdout is a tty. If not, use json or yaml instead, thus allowing the output to be piped to downstream commands:

$ az vm list --output jsonc | jq '.[].name'
"azureclitestlinux"
"azureclitestwin"

$ az vm list --output json > out.txt
$ cat out.txt --show-nonprinting
[
  {
    "additionalCapabilities": null,
    "availabilitySet": null,

ℹ This issue doesn't happen on Windows because ANSI control sequences are removed by colorama.

[jiasli]

Closing as this should go to https://github.com/microsoft/knack