Refactors and reworkd rule core.

Now rule has :one_off field to avoid repeating of processing the same
rule several times (causes incrementing of values).
Rewrights rules to remove {foobar} + some_value things (only true
:one_off rules need this, for ex. {strength} + 1). Now rules should be
written like:

what: "1"
if: "condition"
store_as :foobar

and one_off rules like this:

what: "{some_field_value} + some_value"
if: "condition"
store_as :foobar
one_off true