Add options alwaysShowY and alwaysShowX.

[SvenSchoene]

• Read CONTRIBUTING.md
• Run npm test to make sure it formats and build successfully
• Provide the scenario this PR will address(some JSFiddles will be perfect)
  • I describe the use-case for this below.
• Refer to concerning issues if exist
  • No existing issues regarding this found.

There already is a way to keep the scrollbars always visible -- via CSS, as can be seen in the examples.

However, we have multiple instances of PerfectScrollbar on the pages of our single page app. Adjusting the general CSS is not an option for us.

We needed to be able to control this on an instance level.

I'm sure there might be a better way to implement this. But I'd still at least like to offer this up as a PR. If you feel like this is not a good fit, I fully understand.

Example:

const ps = new PerfectScrollbar(demo, {
  alwaysShowY: true,
});

I read the contributions guide; ran the formatter; re-built the source files so that /dist is updated; updated the typescript file; and added an example.