Introduced skeleton for calling built-in functions
Make functions accept & return values, not expressions
exists() should return 1 for variable that exists
Parse exists() and plug it into the interpreter
Implemented localtime() builtin VimL function

I chose localtime() not so much because it is particularly important,
but because I wanted to get a function with no arguments implemented to
enhance the parsing, and because its implementation was relatively
trivial, allowing me to focus on what I was really trying to do. At this
point, both localtime() and exists() are actually working if you run the
Vim App (I was not parsing the closing parenthesis before).
Implemented escape() builtin VimL function

This seemed liked a good multiple-argument case to start with. Have yet
to do an optional-argument case, but localtime(), exists(arg), and
escape(arg1 arg2) are all working. This is probably going to need some
refactoring, though: some things which are going on in the Interpreter
layer probably need to be pushed down into Vim.
Updated pull request #1506 after feedback
Fixed AppVeyor warning that "params" is reserved

Fixed AppVeyor warning (which did not appear in my build logs on my
machine) that "params" is reserved for future use by F#.
Merge pull request #1506 from Kazark/impl-exists-function

Begin to add support for builtin functions (escape, exists and localtime)