Add flag for accepting single or comma separated input values

[forgedhallpass]

Currently httpx only accepts input through piping or from a file. Adding a new option to accept a single input would be convenient in certain cases (e.g. debugging).

INPUT:
   -u, -target string      input target host to probe

[ariasmn]

Hi!

Wanted to contribute to this amazing project (that a friend of mine heavily uses :+1: ). Just opened a PR for this issue As I commented, there might be a couple of things that need some work. Please let me know in that case!

[ariasmn]

Closed with #759