feat(styles): add prefix and default to variables

[superbiche]

This PR does a few things related to styles which seem necessary for the v4 release:

• [x] Prefix all SCSS variables with $vhd__ to avoid conflicts when project is used with other frameworks
• [x] Use !default to allow overriding values by setting the variables prior to including styles
• [x] Use rgba() for .datepicker__month-day--first-day-selected (fixes #226)
• [x] Add wrapper to center content on demo page
• [x] Prefix all classes with .vhd__ to avoid conflicts with other datepicker libraries
• [x] Use relative units instead of px

I have been too busy to finish it earlier, I'll deal with the items left in the next few days.

[matiasperrone]

I revised the conflicts and solved! The only issue that I've found is that the "disabledWeeksDays" is a enumeration of date's names instead of numbers relative to first day.

[matiasperrone]

But that is not the objective of this PR

[superbiche]

But that is not the objective of this PR

Yup, I guess we can keep this issue scoped to the todo list, but make an issue for the disabledWeeksDays and fix it before v4.0.0.

And thanks for resolving the conflicts !

[matiasperrone]

I agree! 😀

[matiasperrone]

I am working right now on "Prefix all classes with .vhd__ to avoid conflicts with other datepicker libraries"

[matiasperrone]

Some extra testing is necessary. I didn't have the time to finish testing the last bullet.

[superbiche]

@matiasperrone this is HUGE! Thanks so much! I'll take some time tomorrow to test this :+1:

[matiasperrone]

Some extra variables were introduced. But I wasn't able to solve the border of "today".

Some weird issue happens with the number when selected and hover, and out... probably some decimal error...

[matiasperrone]

The testing area is nicer also 😁

[matiasperrone]

And then before going out with the Final Release we have to be able to launch a new doc https://krystalcampioni.github.io/vue-hotel-datepicker/#moreExamples

Is the "gh-pages" branch

[matiasperrone]

I think that I have an idea but note sure yet.

[superbiche]

:tada: