Issue title: [Bug Report] Solo Text Field no longer allows `flat`
 Issue body: ### Versions and Environment
**Vuetify:** 1.1.0-rc.1
**Vue:** 2.5.16
**Browsers:** Chrome 67.0.3396.87
**OS:** Mac OS 10.14.0

### Steps to reproduce
`<v-text-field solo flat></v-text-field>`

### Expected Behavior
The solo box would render without a shadow

### Actual Behavior
https://github.com/vuetifyjs/vuetify/pull/4286 moved the drop-shadow onto `.v-input__slot`, which prevents the `elevation-0` override class from taking effect. 

### Reproduction Link
[https://codepen.io/jeffberry/pen/eKMZjw](https://codepen.io/jeffberry/pen/eKMZjw)

### Other comments
--

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 Comments: 
 Comment 0: https://github.com/vuetifyjs/vuetify/commit/6257e6d511409b54b57c12333def09a31b63080d
{'source': 'GithubIssues', 'repo_name_with_owner': 'vuetifyjs/vuetify', 'issue_id': 334227400, 'repo_stars': 37118.0}