Add class to input

[renatomaric]

Now you can use <datepicker class="form-control"></datepicker> to add class to input: <input readonly="" class="form-control" type="text">

[coveralls]

Coverage remained the same at 99.19% when pulling 41da478 on renatomaric:add-class into 2257a01 on charliekassel:master.

[renatomaric]

@charliekassel: For the class, maybe it's better to put on the div.datepicker element so everything can be styled insider it to user preferences. This would solve #43

I had no issue with custom styling datepicker with .datepicker class, ex.:

<style type="text/css">
      .datepicker input {
        background-color: red;
      }
  </style>

This PR is intended when they using CSS framework (in my case Bootstrap) and only need to add framework's class.

[charliekassel]

Thanks @renatomaric I'll add the wrapper class prop myself.

Btw, this PR still contains renatomaric@e872d88#diff-46c30fb1bb2204a43024aab1429808eb

[vanderlin]

can this merge in...

[coveralls]

Coverage remained the same at 99.19% when pulling c11aa1f on renatomaric:add-class into 05b055d on charliekassel:master.

[charliekassel]

I've added support for this in c8d29bf

[JoshuaDFair]

So after messing with this for a bit a good example of how to style your input is:

<div class="col-md-4">
         <div class="form-group">
              <label>Start Date:</label>
              <datepicker v-model="startDate" input-class="form-control"></datepicker>
        </div>
</div>