Add context! macro to reduce boilerplate when building Contexts

[GandelXIV]

This macro greatly simplifies building Contexts, as you just pass it key-value pairs and inserts them for you.

Example:

use tera::context;
let ctx = context! {
    name => "Brian",
    age => &24
};

Expands to:

let ctx = {
    let mut context = Context::new();
    context.insert("name", "Brian");
    context.insert("age", &24);
    context
};