Accept different kind of modifiers

[gpbl]

This PR extends the type of modifiers values. In addition to a function (day: Date) => Bool, modifiers can be now contain:

• Date objects
• range objects, e.g.

   { from: new Date(2017, 10, 18), to: new Date(2017, 10, 25) }

• after/before objects:

  { after: new Date(2017, 10, 18) }
  { before: new Date() }

• an array of the above.

Using these new values, we can avoid to create new functions at each rendering (see #181) and pre-populate the modifiers with a predefined set of data (see #247).

[coveralls]

Coverage decreased (-0.7%) to 99.291% when pulling 57ba9eb on modifiers-day into e433240 on master.

[coveralls]

Coverage decreased (-0.7%) to 99.291% when pulling 57ba9eb on modifiers-day into e433240 on master.

[coveralls]

Coverage remained the same at 100.0% when pulling f8058e6 on modifiers-day into e433240 on master.