Ability to track a signal's value

[cesarParra]

Add the ability to get a signal's initial value and previous value.

This should be available for computed values as well (since they are signals behind the scenes)

We want to avoid the memory overhead of having all of this information stored, so this should be an optional flag that developers need to opt-into.

Proposed syntax

To be determined. Needs some investigation.

[stanleyzapata]

Actually you already did this in #23 for onError, if you peek before doing the calculation for the computed it can always get the previous value and pass it to the computed function

[cesarParra]

That's not really possible with peek because that would require for the callback function to access the computed value before it has been initialized, so that'd result in a runtime error.

[stanleyzapata]

Couldn't you check if it has been initialized or not, if it has do a peek otherwise return undefined or null

[cesarParra]

Its not really an issue solvable with code, since the problem is that JS will simply consider it invalid code, so it'll throw an error (JS doesn't throw compile errors, so in this case the error will surface at runtime, but in a compiled language that's the kind of thing that would simply not compile).