Allow integer quantities for ingredients

[langston-barrett]

We currently use readFrac to read quantities from recipes stored in JSON/YAML:

instance Yaml.FromJSON Ingredient where
  parseJSON = Yaml.withObject "Ingredient" $ \o -> Ingredient
    <$> (readFrac <$> o .: Text.pack "quantity")
-- snip
readFrac :: String -> Ratio Int
readFrac = -- snip

readFrac requires a string as input, so users must ensure that the quantity field parses as a string, either by unnecessarily adding a denominator, e.g.

    - quantity: 1/1

or by quoting the value:

    quantity: '1'

I think it would be nice to support JSON/YAML integers directly (in addition to fractions), so users could just write:

    quantity: 1

which is more like what I would do in a plain text file.

[Jinbobo]

I'd like to claim this one if no one is working on it

[langston-barrett]

@Jinbobo Go for it!