Add ability to work without begin.

[pdeffebach]

Fixes #17. But I would understand if you choose not to merge it. Because we don't get a new expression on every line, we rely on Julia's parsing to make expressions, meaning we can't use binary or ternary operators, or anonymous functions.

Nonetheless, I think it might be nice for a series of transformations on the same line.

See the docstring I wrote for it

---

@chain(initial_value, args...)

Rewrites a series of argments, either expressions or symbols, to feed the result
of each line into the next one. The initial value is given by the first argument.

In all arguments, underscores are replaced by the argument's result.
If there are no underscores and the argument is a symbol, the symbol is rewritten
to a function call with the previous result as the only argument.
If there are no underscores and the argument is a function call or a macrocall,
the call has the previous result prepended as the first argument.

When using this form of @chain, without a begin block, you cannot use infix
operators, binary operators, or anonymous functions. For example, @chain 1 (_ + 2)
will fail.

Example:

x = @chain [1, 2, 3] filter(!=(2), _) sqrt.(_) sum

x == sum(sqrt.(filter(!=(2), [1, 2, 3])))

[jkrumbiegel]

When using this form of @chain, without a begin block, you cannot use infix
operators, binary operators, or anonymous functions. For example, @chain 1 (_ + 2)
will fail.

Why is that?

julia> dump(:(@chain 1 (_ + 2)))
Expr
  head: Symbol macrocall
  args: Array{Any}((4,))
    1: Symbol @chain
    2: LineNumberNode
      line: Int64 1
      file: Symbol REPL[48]
    3: Int64 1
    4: Expr
      head: Symbol call
      args: Array{Any}((3,))
        1: Symbol +
        2: Symbol _
        3: Int64 2

Looks fine to me, arg 4 is an expression just like you would have in the block form

[pdeffebach]

It has to do with block. quote returns expressions starting with block while :(t + _) does not.

julia> @chain 1 begin 
       (_ + 2)
       end
block = quote
    #= REPL[4]:2 =#
    _ + 2
end
3

julia> @chain 1 (_ + 2)
block = :(_ + 2)

I have fixed all those issues with a small fix just now.

[pdeffebach]

Okay this is ready for a serious review!

[jkrumbiegel]

Thanks! I've just added a small additional test and some docs locally, then merged it here because it was quicker to do so #24