Add range clauses ([NOT] BETWEEN) support

[denisvm]

Implementation of range clauses for BETWEEN and NOT BETWEEN.

sql, _, _ = db.From("test").Where(goqu.L("(a + b)").Between(10,100)).ToSql()
fmt.Println(sql)
// prints: SELECT * FROM "test" WHERE ((a + b) BETWEEN 10 AND 100)

PS: I've not found a clean way yet to implement the expression mapping in mapToExpressionList function.

[doug-martin]

I think you can use the default_and_fragment here

[doug-martin]

So Im torn on this you could go a couple of different ways

1. I think Rhs1 and 2 could be joined into a single slice of what would always be two values here and you could keep with the pattern on Lhs and Rhs and change the between methods to take variadic args but that could get messy.
2. Create a new Range type I("a").Between(Range(1,2)) which could still allow you to get the single Rhs and allow you to include between and notbetween in the goqu.Ex code which was commented out above.

What do you think? I lean towards option 2.

[denisvm]

Yeah, option 2 looks cleaner and less error prone :D

[denisvm]

Hello @doug-martin, is there anything missing to merge this? Thanks!

[doug-martin]

I think we can replace the rhs1, rhs2 with a single RangeVal

[doug-martin]

@denisvm sorry I didnt notice that you had pushed up your changes to address that comment. I think the only thing is merging your rhs1/2 to a single RHS since you have added the RangeVal. Comment once that is done and Ill merge this. Great work!

[TechnotronicOz]

golint gripes about using RangeVal{1,10} this way. I would specify the Start and End like you do in https://github.com/doug-martin/goqu/pull/25/files#diff-7101a3ea121adf8d5b0acb5a2c520ca3R1524

[denisvm]

@doug-martin I've merged both rhs1 and rhs2 values into a single RangeVal struct as you've suggested

[doug-martin]

Thanks again for the PR. I have published under v3.2