Fix #1235 , add one-fifth column

[Naskalin]

fix this #1235 , add one-fifth column
This is really useful, for example, when you need to show more data for width fullHd (but not <div class="column is-2-fullhd"></div> for 6 column in row)

<div class="columns is-multiline">
    <div class="column 
                is-half-tablet 
                is-one-third-desktop 
                is-one-quarter-widescreen 
                is-one-fifth-fullhd"></div>
    <!--column-->
    <!--column-->
    <!--column-->
    <!--...-->
</div>

[jgthms]

You would also need two-fifths three-fifths and four-fifths, like the other classes. But that would add a lot of code, hence why I didn't want to add it in the first place.

[Naskalin]

Ok, I understand, pr updated: add two-fifths three-fifths and four-fifths

[jgthms]

Thanks! Can you update the Changelog and the Docs as well?

[Naskalin]

Synced with master branch. Update Docs and Changelog.