Use signtool.exe on windows

[develar]

I am going to use signtool.exe in my fork, because I don't want to reinvent the wheel and duplicate code, but have to sign windows binaries outside of Squirrel.Windows.

So, it will be natural solution to not add yet another exec in my code, but just add signtool.exe support here, so, all clients of this signcode module will automatically get support of signing on Windows without additional lines of code.

If you don't mind, I will prepare PR.