The option `skipUpdateIcon` was not documented, but seems necessary when using `setupIcon` to avoid error messages. I could only find it in a comment buried in a closed issue (#81 (comment)).

Documented "skipUpdateIcon" option.

- Closes Issue #253
- Packages the .exe.sig file in the nupkg

* Version bump to 1.9.0

* Let's just forget about code pages for the moment

* Update fs-extra to 7.0.0 and remove obsolete fs wrappers
* Update ava and babel
* Replace bluebird with pify
* Require the correct Node version in package.json

Changed the logic in parsing the signWithParams to allow for using them
alongside the certificate options, rather than instead of them.

This stems from times I was debugging the sigining procedure, or
wanted to tweak it, but wished to do so without manually building
the arguments for the certificate file and password as well.

To make this change non-breaking, I added a conditional testing that
the user-provided signWithParams contain neither /f nor /p, and only
in the case that neither of those exists and both certificateFile
and certificatePassword are provided, both are used. Otherwise
the behavior is just like it currently is.

* Update to Squirrel.Windows 1.9.1

* Not sure how this happened, but there should never be Update.exe in here

* Fix the names on the updated Squirrel too