Add an option to set width for pictures

[cang-ming]

As we have already input the name for pic, why not add another option for width?

1. set it a option waiting for manually open for users
2. It should be able to set the default width such as 500px, then do not need to input it everytime
3. If do not use default value, after inserting pic and inputing its name, press 'tab' to the next input blank to set width for convenience.

It will be usefull when writting notes while watching some courses ( too trouble to input latex at the same time),
Thanks a lot!

[jdninqld]

+1

[lxq1]

我们也需要这种功能，很方便，感谢

[j-adel]

This would be amazingly helpful

[VictorJermiin]

I was also just looking for this feature. Would love this!

[BadWinniePooh]

Hi folks,
I think I solved your "problem" with this PR. It's waiting for merge. 😉

Cheers