Feature/set alternate syntax

Maybe this is just me, but I sometimes find the current set tag pair syntax to be hard to read.

```
{exp:stash:set}
  {stash:channel}news{/stash:channel}
  {stash:limit}10{/stash:limit}
{/exp:stash:set}
```

This pull introduces a new, more concise syntax:

```
{exp:stash:set}
  {stash:channel="news"}
  {stash:limit="10"}
{/exp:stash:set}
```

The is the "setter" syntax used by the native preload_replace template tag.

You can also use whitespace before and after the equals sign, so you can line up your "params" if you so desire.

```
{exp:stash:set}
  {stash:channel = "news"}
  {stash:limit   = "10"}
{/exp:stash:set}
```

This was just an idea I had, maybe you'll find it useful. Or maybe not.