Possibility to set default file name to write to using nwsaveas

Feature request from rogerwang/node-webkit#626 and some more. Generally I need that myself.

Now you can propose default filename for user. Examples:

1. The way it was before continues working as well
    ```<input style="display:none" id="save-file-dialog" type="file" nwsaveas>```
    User gets saveAs dialog without any proposed name

2. Proposing name:
    ```<input style="display:none" id="save-file-dialog" type="file" nwsaveas="test.txt">```
    Note `nwsaveas` attribute now has file name.

3. Proposing full file path and it's name — this is actually unplanned bonus… :smile:
    ```<input style="display:none" id="save-file-dialog" type="file" nwsaveas="c:\dev\nw\test.txt">```

Note you still can use nwworkingdir and add new nwsaveas filename proposing (without path) — there is no any compability break.

Compiled and tested on windows 8. Hope you merge it.