allow assignment to underscore (_) without a warning (closes #22)

[chocolateboy]

This uses the same approach as n_ (@borisdiakur++) to ensure that _ can safely be assigned and reassigned without:

• a warning: Expression assignment to _ now disabled.
• resulting in _ becoming undefined

Tested on:

• v4.0.0
• v4.2.1
• v4.8.1
• v6.10.1
• v7.8.0