Added option to generate html without wrapping it in a tag (and a config option)

I wanted to generate translations without wrapping them in a tag, like
```html
<p>{{t 'verb.eat'}} {{t name.carrot}}</p>
```
to yield
```html
<p>Eat carrot</p>
```


So I updated i18n so that setting tagName to false would result in no wrapping:
```html
{{t 'verb.eat' tagName=false}}
```
yields
```html
Eat
```


Also, I wanted to avoid having to repeat it everywhere, so I added an `I18n.config` hash to hold config options, and a `tagName` key, defaulting to `span`.
That way, currend behavior is preserved, yet can be overridden as needed (I get my wish by setting `Em.I18n.config.tagName=false;` in an initializer).
I also updated docs and tests (and fixed a typo in contributing.md).