`left =~ right` return true if left contains right, using Rust's
`String::contains`. `!~` is the negated version.
A new `apply_operator` function is added which decouples evaluation from
`Value::compare`. This returns a `Value` and opens the door to
implementing `+` for example, though it wouldn't be useful immediately.
The `operator!` macro had to be changed slightly as it would choke on
`~` in arguments.
Adds modules for internal, external, and dynamic commands, as well as
the pipeline functionality. These are exported as their old names from
the classified module so as to keep its "interface" the same.
