Electron 7 support

[magne4000]

Description

Installing the package with Electron 7 currently throws on getPassword.
It doesn't even throw an error, it throws the number 32766.

I try to force rebuild with electron-rebuild -f -o keytar but it doesn't seem to do anything.
Custom .npmrc doesn't do the trick either.

[magne4000]

Found a way to force rebuild: yarn run node-gyp rebuild --target=7.0.0 --arch=x64 --dist-url=https://electronjs.org/headers --runtime=electron -C node_modules/keytar
It works after this.

[shiftkey]

@magne4000 I'd take a PR to convert these beta tags for Electron 7 into the stable version (probably also needs to bump node-abi):

node-keytar/package.json

Lines 39 to 40 in 90e995c

	"prebuild-electron": "prebuild -t 4.0.4 -t 5.0.0 -t 6.0.0 -t 7.0.0-beta.3 -r electron --strip",
	"prebuild-electron-ia32": "prebuild -t 4.0.4 -t 5.0.0 -t 6.0.0 -t 7.0.0-beta.3 -r electron -a ia32 --strip",

EDIT: looks like #232 should cover the node-abi support needs

[magne4000]

@shiftkey here you go! #233

[shiftkey]

@magne4000 I've published this up to NPM with 5.0.0-beta4 and I'll promote this to 5.0.0 later in the week. Details on the breaking changes are listed here: https://github.com/atom/node-keytar/releases/tag/v5.0.0-beta.0