Add option to set the executable name separate from the app name

[malept]

• I have read the contribution documentation for this project.
• I agree to follow the code of conduct that this project follows, as appropriate.
• The changes are appropriately documented (if applicable).
• The changes have sufficient test coverage (if applicable).
• The testsuite passes successfully on my local machine (if applicable).

Summarize your changes:

Adds the executableName option (--executable-name in CLI) which allows to set the executable name, which defaults to name but does not affect the .app name.

Example:

const packager = require('electron-packager')

packager({
  name: 'App Name',
  executableName: 'app-name',
  platform: 'all'
})

On Windows, the Electron binary will be app-name.exe. On Linux, the Electron binary will be app-name. On macOS, the app is App Name.app and the binary inside it is App Name.app/Contents/MacOS/app-name.

---

Fixes #745.

[MarshallOfSound]

Seems legit 👍