Add =~ and !~ operators on strings

`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.

crates/nu-source/src/lib.rs

@@ -11,5 +11,6 @@ pub use self::meta::{
 pub use self::pretty::{
     b, DebugDoc, DebugDocBuilder, PrettyDebug, PrettyDebugWithSource, ShellAnnotation,
 };
+pub use self::term_colored::TermColored;
 pub use self::text::Text;
 pub use self::tracable::{nom_input, NomSpan, TracableContext};