improve parser for model strings

[chriscoey]

see comments on JuMP-dev gitter. it would be nice if a scalar affine function such as 1.2x - y + 1 would parse in a model string (or at least throw more helpful error messages). currently it has to be written as 1.2x + -1y + 1 to parse.

[odow]

Leaving a breadcrumb for future readers. Here's where to start:

MathOptInterface.jl/src/Utilities/parser.jl

Lines 109 to 110 in f08c78c

	# only accept Expr(:call, :+, ...), no recursive expressions
	# TODO: generalize. x - y + z would be useful