Introduce Catmandus less_than and greater_than conditionals

[TobiasNx]

Introduce greater_than and less_than conditionals from Catmandu.
Requested by TU D for comparing numbers and years. This seems to be a good feature and already supported by catmandu.

from Catmandu:

# greater_than(X,Y) is true when X > Y
if greater_than('year','2018')
 add_field('my.funny.title','true')
end
# greater_than on arrays checks if all values are X > Y
if greater_than('years.*','2018')
  add_field('my.funny.title','true')
end

and

# less_than(X,Y) is true when X < Y
if less_than('year','2018')
 add_field('my.funny.title','true')
end
# less_than on arrays checks if all values are X < Y
if less_than('years.*','2018')
  add_field('my.funny.title','true')
end

[blackwinter]

Functional review: @TobiasNx
Code review: @fsteeg