Adding ability to pass boolean attrs to link helper methods

[jwoertink]

Purpose

Fixes #1029

Description

You can already use the a() method and pass boolean attrs, but now this PR lets you pass those in the link methods as well.

Checklist

• - An issue already exists detailing the issue/or feature request that this PR fixes
• - All specs are formatted with crystal tool format spec src
• - Inline documentation has been added and/or updated
• - Lucky builds on docker with ./script/setup
• - All builds and specs pass on docker with ./script/test

[paulcsmith]

Looks good!