feat(inputs): columns are now optional if column = 1
feat($core): adds support for input's placeholders

The placeholder can be set using the `input_html` options
or with a locale in the `awesome_form.placeholders` scope.
refactor(wrappers): extracts field's errors in a partial

Arbitrary partials can be rendered using the `render_partial`
method of the builder.
feat($core): adds support for input's hints

Hints are retrieved from locales or can be set using the
`hint` option.
The hint will be rendered using the `wrappers/_hints` partial.