Make border classes more atomic-friendly

[thierryk]

We have the following helper classes:

.Bd-0 {
    border: 0;
}
.Bd-1,
.Bdx-1,
.Bdy-1,
.Bdt-1,
.Bdend-1,
.Bdb-1,
.Bdstart-1 {
    border-width: 0;
    border-style: solid;
}
.Bdstart-1 {
    border-left-width: 1px;
}
.Bdend-1 {
    border-right-width: 1px;
}
.Bdx-1 {
    border-right-width: 1px;
    border-left-width: 1px;
}
.Bdy-1 {
    border-top-width: 1px;
    border-bottom-width: 1px;
}

But I think we should move all this to rules.js — using classes like Bdw-, Bds-, Bdc-.
That way users can create border styling without having to create custom Atomic classes.

Thoughts?

[src-code]

PR #74 already adds most of this, just needs the Bdy and Bdx support added.

[src-code]

Resolved in #74

[thierryk]

For the record: helpers.css (from another repo) has been updated with border classes meant to help when styling boxes with a single pixel border but those rules need to integrate Atomizer somehow.

[src-code]

@thierryk I'll add a new issue for that.