Display changelog of updated plugins

[kwbr]

I wonder if it's possible to list the changes of the updated plugins on update:

cbdf10a Fix #573 - Update the README to recommend installing via the built-in package system
0b50ebb Fix #779 - Handle empty output for tslint
235fc90 Fix #308 - Check Dart files with dartanalyzer
aa94d09 Fix #710 - Show hlint suggestions as info items, and include end line and column numbers
5a6ffc2 Add a missing test file

[hauleth]

Preferably in quickfix or loclist.

[k-takata]

I'm wondering what is the best UI for this.
Currently, minpac uses :echom to show the results, and it can be seen by :messages later. This is actually almost enough for me, and I don't want to pay much cost for creating fancy UI.

[hauleth]

@k-takata maybe quickest/loclist?

[kwbr]

:messages would be enough for my taste as well.

[k-takata]

Fixed by #58.