Make argon2 optional

Resolves #26.

Since dirs is a reverse dependency of this crate and is used an awful
lot, it's nice to have as small a dependency footprint as possible. dirs
doesn't need the authentication stuff so we can drop a large number of
deps. I imagine this will probably be useful in other contexts as well.
5 jobs for master in 1 minute and 24 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #29518
build:linux

00:00:27

passed #29519
build:redox

00:00:25

 
  Test
passed #29522
test:linux

00:00:33

failed #29521
test:redox

00:00:24

failed #29520
test:linux

00:00:26

 
Name Stage Failure
failed
test:redox Test
} to /root/.redoxer/base
* Requesting https://static.redox-os.org/pkg/x86_64-unknown-redox/ca-certificates.sig
Installing package ca-certificates
* Failure 404 Not Found
thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: Custom { kind: NotFound, error: "https://static.redox-os.org/pkg/x86_64-unknown-redox/ca-certificates.sig not found" }', /root/.cargo/registry/src/github.com-1ecc6299db9ec823/redox_installer-0.2.7/src/lib.rs:124:38
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
error: test failed, to rerun pass '--lib'
redoxer cargo: exit code: 101
ERROR: Job failed: exit code 1