Add support for G-Mail "+" Addressing

[timlight10]

Hi, is there a way to add support for G-Mail addresses with + signs in them? This is a valid use of G-MAil addresses and is very helpful for filtering, etc..

For example, if my e-mail is my.email@gmail.com, I can use my.email+abantecart@gmail.com, my.email+junk@gmail.com, etc... All e-mails will actually be delivered to my.email@gmail.com

However, if you enter one of these "+" addresses into Abantecart, you get an error message back:

App Error E-Mail Address does not appear to be valid!

[abolabo]

you can change regex pattern and add + sign in this line

https://github.com/abantecart/abantecart-src/blob/master/public_html/core/init.php#L71