[Feature] Initial concept for adding percentage based goals (#858)

This is an initial concept for adding percent based goal targets.

This version works by using a string in the form of: 
#template 10% of Income <- Income is an income category name. Only
income category names will work here currently.
or
#template 10% of all income<- 'all income' is a keyword in this context
and will base the calculation on the total income for the month.

Some of the nicer touches like Jed's polite notification that the syntax
isn't correct is not implemented here yet.