Print a final report suggesting to open an issue on rust repo

[spastorino]

This PR make the tool output a final report.

The report looks like ...

==================================================================================
= Please open an issue on Rust's github repository                               =
= https://github.com/rust-lang/rust/issues/new                                   =
= Below you will find a text that would serve as a starting point of your report =
==================================================================================

# Regression found in the compiler

searched nightlies: from nightly-2019-09-29 to nightly-2019-09-30
regressed nightly: nightly-2019-09-30
searched commits: from https://github.com/rust-lang/rust/commit/488381ce9ef0ceabe83b73127c659e5d38137df0 to https://github.com/rust-lang/rust/commit/8431f261dd160021b6af85916f161a13dd101ca0
regressed commit: https://github.com/rust-lang/rust/commit/0bbab7d99dde8620604fb265706dc8bff20345a7
source code: URL OF A REPOSITORY THAT REPRODUCES THE ERROR

## Instructions

PLEASE GIVE US INSTRUCTIONS ON WHAT YOUR APPLICATION DOES
## Error

<details><summary>COLLAPSIBLE ERROR STACKTRACE</summary>
<p>

PASTE THE ERROR YOUR APP GIVES HERE (run with RUST_BACKTRACE=1)

</p></details>

And that pasted on an issue looks like spastorino/cargo-bisect-sample#1

r? @Mark-Simulacrum