Extract ldiff display logic to reuse it as a lib

[Annih]

The ldiff "display" logic was too tightly coupled with the command line program. See #46.
Let's just extract it in a distinct class method accepting all the necessary parameters.

One can now use:

Diff::LCS::Ldiff.diff?(
        Diff::LCS::Ldiff::InputInfo.new('file1'),
        Diff::LCS::Ldiff::InputInfo.new('file2'),
        :unified,            # or :ed or :context or :report
        $stdout,           # or a ref to [] or to a StringIO or a File
        binary: nil,        # or true or false if you want to be explicit
        lines: 3)            # to customize the number of context lines

The result will be true in case of diff and false otherwise.
The output if any, will be appended to the output parameter.

I also used the opportunity to refacto the format handling to use a "nice" switch instead of multiple if statements.

Note: I didn't add test here, as normally the ldif bin specs should cover the critical use cases.

This should fix #46 🤞🏻

[halostatue]

This should be a namespaced constant to not leak constants to the global namespace, and I would replace :mtime with :stat and just grab .mtime from the stat when we need it (the rest of the stat data may be useful for others in the future).

Suggested change

	InputInfo = Struct.new(:filename, :data, :mtime) do
	def initialize(filename)
	super(filename, ::File.read(filename), ::File.stat(filename).mtime)
	end
	end
	Diff::LCS::Ldiff::InputInfo = Struct.new(:filename, :data, :stat) do
	def initialize(filename)
	super(filename, ::File.read(filename), ::File.stat(filename))
	end
	end

[halostatue]

I have merge #104 which will hopefully rebase into this nicely.