feat(select): allow auto-width based on placeholder / value

### fixed width on mat-form-field
I want my select to have the width of the selected item, fixed width on mat-form-field makes it impossible, you can override `.mat-form-field` to have `width` `auto`, along with overriding `.mat-select-value` to be `width` `auto` and `max-width` `100%`, but then you have a problem, because placeholder is absolutely positioned, which means that the select without any value set yet will be zero width (actually 18px because of the arrow).

https://stackblitz.com/edit/material2-select-width-issues?file=styles.scss

Now I could also override placeholders not to be absolutely positioned, but that's not gonna work properly either, because the arrow is inside the `md-select` element, while the placeholder is outside


I'm not sure if that 200px width is something that's on material spec, but the DOM could perhaps be structured a bit differently, in a way that wouldn't prevent devs from achieving fluid width (which is a bit closer to how native selects work)

## different height when selected vs. placeholder

cc @crisbeto

Comience con la reproducción vinculada de StackBlitz e inspeccione la estructura de mat-form-field y mat-select implicada en los casos de fixed-width y placeholder. Compare el comportamiento actual con el comportamiento auto-width solicitado, tanto para un placeholder vacío como para un valor seleccionado; se considera terminado cuando el select puede ajustar su tamaño de forma fluida sin romper el diseño de la flecha ni del placeholder.

Escrito por el modelo de indexación a partir del texto del issue.