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

从链接的 StackBlitz 复现开始，检查涉及 fixed-width 和 placeholder 情况的 mat-form-field 与 mat-select 结构。将当前行为与所请求的 auto-width 行为进行比较，分别针对空 placeholder 和已选值；完成的标准是 select 能够灵活调整大小，同时不破坏箭头或 placeholder 的布局。

由索引模型根据 Issue 内容生成。