Adds hook scope aliases `example` and `context`

This allows for nicer, more sensible naming in the context of `RSpec.configure` blocks and elsewhere.

For example, this block sounds like it's a hook that runs once, before any example:

```ruby
RSpec.configure do |config|
  config.before(:all) do       # ... confusing :(
    # do something
  end
end
```

But it's actually a hook that runs once before every _context_, which is confusing. So a better name would be:

```ruby
RSpec.configure do |config|
  config.before(:context) do   # sensible!     :D
    # do something
  end
end
```

This change adds, in hook scopes, the ability to use `:example` as an alias for `:each` and `:context` as an alias for `:all`.

See #297 for further discussion.