Add stdin function

[sevagh]

Allows you to put the contents of a String in the program's stdin.

Fixes #26

[epage]

Looks great!

Two things

• Unsure about @killercup but I try to keep the commit history clean in my projects. Unless killercup comes in and says otherwise, do you mind cleaning up the commits?
• Name. As I said in Piping (stdin) to the process #26, I personally do not (yet) have opinions on the name (with_stdin, stdin, given_input). Not sure if @killercup does. I'd be willing to submit this as-is and release it, as long as the name is settled before 1.0 (Future of assert_cli #41).

[killercup]

Unsure about @killercup but I try to keep the commit history clean in my projects. Unless killercup comes in and says otherwise, do you mind cleaning up the commits?

@killercup agrees and is in favor of squashes.

Name.

Hmmm. Do we have an existing pattern? Do the API guidelines give us a pattern to use?

[sevagh]

I'll squash the commits. I was more curious about other feedback, i.e. is a String OK? I tried something with a Read trait but that involves either boxes or lifetimes and the change became too intrusive.

[epage]

I'm not seeing anything in the naming guidelines that might apply.

Command configures through

• stdin
• `stdout
• stderr

For assert_cli, we cuerrently have

• stdout
• stderr

So if we consider that precedence, then the name would be just stdin. Overall, I'm not seeing a big deciding factor in any direction.

[sevagh]

I renamed it to stdin() and squashed the commits.

[sevagh]

Also not sure if I should be running cargo fmt - it makes a lot of changes in files I haven't touched.