New disruptive API change for V3: Using ENV

[suntong]

Don't know how many of you are using the .ENV to access environment variables (e.g., {{.ENV.SHELL}}), but because of the difficulties fixing #17, to make easygen accepting any arbitrary data again, the cleanest solution is to turn .ENV into a function.

So in V3, instead accessing environment variables like e.g., {{.ENV.SHELL}} use {{ENV "SHELL"}} instead.

[suntong]

@grrtrr, please take a look at this and tell me if it'll break your code.

FYI, I've checked in the change into a branch, https://github.com/go-easygen/easygen/commits/v3
together with the CLI change you raised.

[grrtrr]

No, that is fine, thank you for keeping on improving this tool.

[suntong]

Thanks Gerrit, I'll merge it in then.