Add constructors that infer the metadata types from the data values provided to constructor

[CameronBieganek]

The current MetaGraph constructor requires the user to explicitly specify the types of the metadata via the arguments Label, VertexMeta, and EdgeMeta. That seems a little unusual to me. It's much more common for constructors to determine the values of type parameters based on the data passed to the constructor, e.g.

julia> [1, 2]
2-element Array{Int64,1}:
 1
 2

julia> ["hello", "world"]
2-element Array{String,1}:
 "hello"
 "world"

It would be nice if there were some constructors like the following that infer the type parameters from the data:

MetaGraph(
    SimpleGraph(),
    vertices = [(:a, :color => "blue"), (:b, :color => "red")],
    edges = [(:a, :b, :linewidth => 2)]
)

The fallback for creating a typed but empty graph would be something like the following:

MetaGraph{SimpleGraph{Int}, Symbol, Dict{Symbol, String}, Dict{Symbol, Int}}()

which corresponds to how one normally creates a typed but empty object. E.g., for a typed but empty dictionary, you would use this:

Dict{Symbol, String}()

Or for a typed but empty vector:

Vector{Int}()

...although for arrays we have the sugar Int[].

[bramtayl]

This is a great idea!

[gdalle]

We don't have lots of constructors yet, but this seems like the logical next step! Thanks for the idea

[gdalle]

@CameronBieganek following up on this, do you want to draw up a PR?

[gdalle]

Side note: this is actually essential for our type stability claim, cause the constructor with types as kwargs is not inferrable