syntax checker complains about non-existant in at for loops

[mezzodrinker]

for k,v in next,_G do
  -- do something
end

The syntax checker complains about a missing in at the for loop even though it's there. Need to add in to mts.yml, I probably forgot.

[mezzodrinker]

added in to keywords list, resolves #30

→ <<cset 38c506e469f2>>