Number Fields use String value storage; allow step="any" attribute

[mattdb]

Note: this PR requires bumping the package version, I believe it can go with the rest of the 8.0 changes currently in master. Should it miss 8.0, a new version bump would need to be added.

As discussed in #25, storing number fields' values as Maybe Floats caused issues when entering decimal values. This updates number fields' value storage to use String.

Also related to decimals in number fields, the HTML spec allows the step attribute to be set to "any" to allow arbitrary precision. (If the step attribute is not present on the number input, the default is a step value of 1 (Integers only)). Making use of the "any" value is of interest to our app.

Accordingly, I changed the step item in NumberField.Attributes from number to Maybe number-- The idea being specifying Nothing means any step value is allowed.

Step could probably be more elegantly and clearly modeled with a custom type — do we want to introduce that level of complexity? Something like:

type NumberStep number
    = AnyStep
    | Step number

I feel like using Maybe is consistent with the rest of the Api design, but let me know if you prefer the custom type, I'm happy to add it if you'd prefer.

[hecrj]

This is great! Thank you!

I think a step of Nothing being translated to any makes sense. In the end, any means no step whatsoever.

Also, thank you for updating the changelog. I will release 8.0 soon!