RFC acceptance process: we should hyperlink issue/PR numbers

[lilyball]

When an RFC is accepted, it is modified to include the PR number and the issue number for the tracking issue on rust-lang/rust. Since RFCs are markdown rather than plain text, these two numbers should be hyperlinked to the actual PR/issue they refer to.

For example, when RFC 18 was accepted (in b683d2e), it was modified like so:

-- RFC PR #:
-- Rust Issue #:
+- RFC PR #: 79
+- Rust Issue #: 14309

This should instead have been:

-- RFC PR #:
-- Rust Issue #:
+- RFC PR #: [79](https://github.com/rust-lang/rfcs/pull/79)
+- Rust Issue #: [14309](https://github.com/rust-lang/rust/issues/14309)

[chris-morgan]

Duplicate of #62.

[lilyball]

You're right.