Make explicit labeler use label-before and label-after classes specifying the label position

Forme doesn't automatically add colons when using the explicit
labeler, it assumes they will be added via a CSS :after content
setting.  However, in general you'll only want to add colons if
the label comes before the input, not after.  So this makes it
easy to use the appropriate CSS selector to only add the colons
where they should be added.
Support :error_attr option for inputs, for setting attributes to use for error message span
When using :grid option to subform in the Sequel plugin, :inputs_opts can be used to pass options to the InputsWrapper

Previously, there was no way to pass explicit options to the
InputsWrapper when using :grid, so you couldn't set specific
classes on the table created.
Support pg_array_to_many associations in the Sequel plugin, treating them similarly to other *_to_many associations

These weren't supported before because Forme doesn't handle
unrecognized association types.  The only code change is that
the field used should be the :key option for the association,
since that specifies the column containing the array of
associated values.
Update README for recent feature additions
Bump version to 1.2.0