Improve handling of escaped strings with String.Format

[Fryguy]

String.Format doesn't handle string with escaped characters properly. There are some pending specs committed demonstrating the problems, namely

• floe/spec/workflow/intrinsic_function_spec.rb

  Lines 59 to 64 in a2bd4ee

  	it "with args with escaped characters" do
  	pending "better handling of escaped characters in the template"
  	result = described_class.value("States.Format('This value is {}', '\\'')")
  	expect(result).to eq("This value is \\'")
  	end

• floe/spec/workflow/intrinsic_function_spec.rb

  Lines 81 to 86 in a2bd4ee

  	it "fails with invalid templates" do
  	pending "better handling of escaped characters in the template"
  	expect { described_class.value("States.Format('\\{}')") }.to raise_error(ArgumentError, "Invalid template in States.Format (matching '{' not found for '}')")
  	expect { described_class.value("States.Format('{\\}')") }.to raise_error(ArgumentError, "Invalid template in States.Format (matching '}' not found for '{')")
  	end