<kbd> style corrections

[priiduonu]

Right now the <kbd> tags seem a little "broken", see:

Maybe add something like:

border: 1px solid #adb3b9;
line-height: 1.4;

to achieve:

[squidfunk]

Fixed in f7fddb0 via #28. This brings the styles more up to the modern look:

[squidfunk]

Released as part of 0.0.2 – will be integrated into Zensical 0.0.13.