feat: loop through workspace members

[Frederick888]

Closes #58. Rebasing and squeezing needed before merging.

Example 1:

⇒  cargo run -- outdated -m ~/Programming/Rust/Rocket/Cargo.toml
    Finished dev [unoptimized + debuginfo] target(s) in 0.0 secs
     Running `target/debug/cargo-outdated outdated -m /home/frederick/Programming/Rust/Rocket/Cargo.toml`
rocket
================
Name               Project     Compat  Latest   Kind    Platform
----               -------     ------  ------   ----    --------
base64             0.6.0       ---     0.7.0    Normal  ---
hyper              0.10.13     ---     0.11.2   Normal  ---
hyper-sync-rustls  0.3.0-rc.1  ---     0.2.1    Normal  ---
idna               0.1.4       ---     Removed  Normal  ---
libc               0.2.31      ---     Removed  Normal  ---
log                0.3.8       ---     Removed  Normal  ---
matches            0.1.6       ---     Removed  Normal  ---
mime               0.2.6       ---     0.3.4    Normal  ---
num_cpus           1.6.2       ---     Removed  Normal  ---
percent-encoding   1.0.0       ---     Removed  Normal  ---
traitobject        0.1.0       ---     Removed  Normal  ---
typeable           0.1.2       ---     Removed  Normal  ---
unicase            1.4.2       ---     2.0.0    Normal  ---
url                1.5.1       ---     Removed  Normal  ---
version_check      0.1.3       ---     Removed  Build   ---

todo
================
Name                 Project  Compat  Latest   Kind    Platform
----                 -------  ------  ------   ----    --------
diesel               0.14.1   ---     0.16.0   Normal  ---
diesel_codegen       0.14.0   ---     0.16.0   Normal  ---
diesel_infer_schema  0.14.0   ---     0.16.0   Normal  ---
quote                0.3.15   ---     Removed  Normal  ---
r2d2-diesel          0.14.0   ---     0.16.0   Normal  ---
syn                  0.11.11  ---     Removed  Normal  ---
synom                0.11.3   ---     Removed  Normal  ---
unicode-xid          0.0.4    ---     Removed  Normal  ---

Example 2 (https://github.com/rust-lang/rust/tree/master/src): https://gist.github.com/Frederick888/82b50aedc3a9d67bc5d43ff345750e26