QuickInstall support (#94)

See this issue: https://github.com/alsuren/cargo-quickinstall/issues/27

Quick Install is a hosted repo of built crates, essentially. The approach I've taken here is
a list of strategies:

1. First, we check the crate meta or default and build the URL to the repo. Once we have
   that, we perform a `HEAD` request to the URL to see if it's available.
2. If it's not, we build the URL to the quickinstall repo, and perform a `HEAD` to there.

As soon as we've got a hit, we use that. I've built it so it's extensible with more strategies.
This could be useful for #4.

This also adds a prompt before downloading from third-party sources, and logs a short
name for a source, which is easier to glance than a full URL, and includes a quick refactor
of the install/link machinery.

Cargo.lock

@@ -32,6 +32,17 @@ version = "1.0.53"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "94a45b455c14666b85fc40a019e8ab9eb75e3a124e05494f5397122bc9eb06e0"
 
+[[package]]
+name = "async-trait"
+version = "0.1.52"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "061a7acccaa286c011ddc30970520b98fa40e00c9d644633fb26b5fc63a265e3"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "syn",
+]
+
 [[package]]
 name = "atty"
 version = "0.2.14"
@@ -114,6 +125,7 @@ name = "cargo-binstall"
 version = "0.5.0"
 dependencies = [
  "anyhow",
+ "async-trait",
  "cargo_metadata",
  "cargo_toml",
  "crates-index",
@@ -133,6 +145,7 @@ dependencies = [
  "tempdir",
  "tinytemplate",
  "tokio",
+ "url",
  "xz2",
  "zip",
 ]