allow to specify alternative keys for some field types

[tyrasd]

Allows to specify additional alternative keys for text, number, tel, email and url fields.

Necessary to properly define tag keys for openstreetmap/id-tagging-schema#905.

Example

{
    "key": "email",
    "keys": [
        "email",
        "contact:email"
    ],
    "type": "email",
    "placeholder": "example@example.com",
    "label": "Email"
}

Todo:

• update docs
• release as v6.3 (this change is backwards compatible, so a v7 is not necessary)