Feature/adding all values in images enum

[HEYGUL]

Add an array in the images enum so we can iterate over them.

[AliSoftware]

Hey, thanks for the PR !

I think that feature is very rarely useful (I never had to iterate over all the images in my Assets Catalog, and don't think most of people do), so I think that'll make the generated file quite long (like when people have 100+ images in their assets catalog)

So here's what I suggest:

1. Keep the existing template unchanged (and revert the examples in README & Playground)
2. Create a separate template that adds what you did so we can have this allValues entry, but as an opt-in option. People wanting to have this could simply use your template instead of the default one.
3. I'll then include your template alongside mine when SwiftGen is installed. Once Store custom templates in ApplicationSupport #42 is merged, which would allow people to use --templateName to specify a template by name instead of by full path, this would make it easier for people to choose the default template without allValues or your template with it

[AliSoftware]

Merged via d430f56

[HEYGUL]

Thank you very much to accept this MR :-) 👍

[AliSoftware]

Now that #50 is merged, you'll be able to use swiftgen images -t allvalues PATH in order to use this specific template which will be bundled with SwiftGen in the next release.