Support piping scripts through stdin

[danobi]

For example, you can now do:

echo 'BEGIN { printf("hello world!\\n") }' | sudo bpftrace -

In other words, you can pipe input to bpftrace now. It's always good to
let people script tools better.

Checklist

• Language changes are updated in docs/reference_guide.md
• User-visible and non-trivial changes updated in CHANGELOG.md