Suppress `col` className in `Col` component

Issue description

• components: Col
• reactstrap version #5.0.0-alpha.4
• bootstrap version #4.0.0-beta.2

What is happening?

Sometimes it’s not desirable to have a column element having the class col. For example, one might want to reproduce the Stacked to horizontal example from Bootstrap’s documentation.

In my specific case, I wan my columns to have only the class col-md-4. If I use <Col md='4'>My content</Col> the resulting HTML element will have the class col col-md-4. By looking into Col’s source code, I found out that I can get rid of the col class by setting xs={false} md='4', but I believe that’s not the correct way of doing it (right?).

I think it would be nice to have an “intended” way of suppressing the col class name.

[pravdomil]

Yes, that’s what I would expect from Col.

<Col md={6}> should create <div class="col-md-6">

<Col md={6} xs> should create <div class="col col-md-6">

<Col> should create <div class="col">

Does it makes sense for everybody?

[TheSharpieOne]

col is in bootstrap’s documentation and features things like <div class="col col-lg-2"> (showing col in conjuction with a col-breakpoint value) I’m fine with making this change if everyone thinks it is the right thing to do. Breaking changes don’t matter, we have semvar.