Invert control of loading

**Is your feature request related to a problem? Please describe.**

Currently, the future is executed soon as the widget is rendered. Should allow for more fine grained control of loading.

**Describe the solution you'd like**

Add a `loadOnMount` property
- Triggers whether or not the future is executed on default
- Default to **true**

Add a `load` function to builder props.
- Triggers load / reload future when executed

**Describe alternatives you've considered**

N/A

**Additional context**

https://github.com/jxom/react-loads#loadonmount