Adding custom and masked password fields in the AP Config page for custom password variables - Solution to issue #33

[Valdez-x]

This pull request solves issue #33.

-Adds a new WiFiSettingsClass::passwordstring and WiFiSettingsPasswordString struct that are copies of WiFiSettingsClass::string and WiFiSettingsString struct but has the html value hardcoded to the value ##**##**##** and the html value is not updated when a new value entered.

-for loop at line 410 checks the args if the custom password field already has the mask ##**##**##** so that it only stores a new value if the value has been changed from the mask ##**##**##**

[Juerd]

Thanks for the PR. I've left a few comments for improvements.

I'm not entirely sure about the name passwordstring because that's very long. I want to wait a day to see if we can come up with something shorter that would still be sufficiently descriptive. Unfortunately password is taken already :P

[Valdez-x]

Sounds good Juerd.

Would also be nice if on the first run the box was completely blank instead of starting with the mask already there.