Allow Feed URL to be parsed with Craft::parseEnv()

[jamesmacwhite]

Description

There might be some use cases where the Feed URL field could benefit from being environment variable aware. Here's a couple of use cases:

1. You have a staging and live environment which use a different feed endpoint per environment and you want to be able to set this without having to modify the feed properties or use a module and call a custom event i.e. EVENT_BEFORE_FETCH_FEED

2. A feed that may require a sensitive token, which you don't want in plain text and instead as a env var e.g.

http://examplesite.com/feed/data.json?token=$MY_SECRET_TOKEN

Currently you would have to use a module with a custom event to modify the feedUrl, as well as manually parse any env var to pass to Feed Me.

[jamesmacwhite]

Aliases are supported, but not env var's I don't think.