Adding filterIf and filterOpt utility methods to Select

[NPCRUS]

fixes #63

[aboisvert]

Code looks good overall. I just wonder about the ergonomics of filterOpt.

Thinking about some alternatives ...

// Currently proposed
ShippingInfo.select.filterOpt(someOpt)((value, table) => table.buyerId `=` value)

// Option 1: filterIf is all we need
ShippingInfo.select.filterIf(someOpt.isDefined)(_.buyerId `=` someOpt.get)

// Option 2: filterOpt doesn't provide unwrapped value
ShippingInfo.select.filterOpt(someOpt)(_.buyerId `=` someOpt.get)

// Option 3: Reverse the parameter order (table, value) instead of (value, table)
// (It seems to me table should come first also because it's the main object being 'carried over')
ShippingInfo.select.filterOpt(someOpt)(_.buyerId `=` _)

The aim is not necessarily code-golfing as it is to help produce easy-to-read queries.

And an even more drastic change would be to make Select a monad so it can be used in for-comprehensions and which could improve ergonomics but that's a more involved change that would require some experimentation to get the combinators right.

[NPCRUS]

@aboisvert you are right, order in option 3 is better

[lihaoyi]

I think this looks fine, happy to merge it when you are done

[NPCRUS]

@lihaoyi im happy and could you pls make a minor release?

[lihaoyi]

Tagged 0.1.17