Merge #1821

Merge #1821

1821: Macro completion tweaks r=matklad a=SomeoneToIgnore

Thanks @uHOOCCOOHu for making the macro completion happen :)

I've added a few tweaks to the current completion to make it a bit more convenient:

* Automatically add braces and put the editor cursor inside of them:
<img width="159" alt="image" src="https://user-images.githubusercontent.com/2690773/64737220-022b9f00-d4f5-11e9-8088-76d4678921ab.png">

Currently I have to add the braces manually which is a bit cumbersome.
One further improvement can be to detect if macro accepts no parameters and place the cursor differently for this case.

* Add an exclamation mark to the macro completion label

This helps to distinguish macros from other completion items and also allows to show only macros in completion if you type `!`:
<img width="722" alt="image" src="https://user-images.githubusercontent.com/2690773/64736987-8b8ea180-d4f4-11e9-8355-2ce4f83b7aa8.png">

<img width="732" alt="image" src="https://user-images.githubusercontent.com/2690773/64737214-ffc94500-d4f4-11e9-946e-1ba2db1c7fb1.png">

Additionally, automatic formatting hooks had adjusted two `help.rs` files, I've added them as a last commit to the PR even though they are not really related.

Co-authored-by: Kirill Bulatov <mail4score@gmail.com>