Add sass variable defaults

[danieljb]

Add !default flag to variable definitions to allow manipulation from an external SASS file.

It would then be possible to load and customize src/balloon.scss directly from another scss file:

/* customstyle.scss */
$balloon-bg:             fade-out(#F00, .25);
$balloon-base-size:      16px;

@import "~balloon-css/src/balloon";

[willylatorre]

Yes please, could this be merged?

[kazzkiq]

I was afraid of accepting this and having no equivalent in LESS. But it seems LESS solution is even easier than SASS, so no changes needed.

Accepting this PR. Thanks for the effort!