Add support for 'is.email'

[joeltankam]

Checks if the given value is an email.

is.email('very.common@example.com'); // true
is.email('other.email-with-hyphen@example.com'); // true
is.email('user.name+tag+sorting@example.com'); // true
is.email('john..doe@example.com'); // false

Checkout full email description on wikipedia.

Update README.md: under is > RegExp

[oaguinaga]

I would like to help with this

[joeltankam]

You're welcome

[joeltankam]

RFC5322 official standard