Allow set operation to take a lambda

[jrmhaig]

The set pre-processing operation will currently set a field to a static value. Issue #112 suggests allowing the field to be set according to a parameter passed to ameoba_dup.

set can now be defined to use the return value of a lambda and keyword arguments can be used on amoeba_dup to specify arguments to the lambdas:

class Test < ActiveRecord::Base
  amoeba do
    set test_field: ->(arg:) { arg }
  end
end

t1 = Test.new
t2 = t1.amoeba_dup(arg: 123)
t2.test_field
# => 123

Note that the @options instance variable on the macro classes has been removed. It is not at all clear what was the purpose of this instance variable as it did not appear to be set anywhere and its removal has not affected any of the unit tests.

[coveralls]

coverage: 93.38% (-1.9%) from 95.324%
when pulling b1b59e7 on jrmhaig:amoeba_params
into 6adbbc5 on amoeba-rb:master.

[coveralls]

coverage: 93.38% (-1.9%) from 95.324%
when pulling ff08351 on jrmhaig:amoeba_params
into 6adbbc5 on amoeba-rb:master.