Add sortAlphabetically option

I would appreciate adding of `sortAlphabetically` option, which would be set to `true` in default (to be compatible with older versions). That's because I need special CSS attributes for older IEs starting with underscore to be defined **after** their standard relatives, but inline-css sorts them **before** their standard relatives. That means the standard attribute overwrites its behaviour.

Before inlining:
```
div {
width: auto;
_width: 230px;
}
```

After inlining:
```

```
which means that even in old IEs `230px` is overwritten by `auto`.