Add responsive font controls

[boldgrid-support]

Is your feature request related to a problem? Please describe.
Some Headings are too large when viewed on a mobile screen and there is no option to set specific font sizes for various devices.

Describe the solution you'd like
Add controls in the Customizer to allow for site wide defaults to control fonts across the range of devices.

Describe alternatives you've considered
Currently you need to use custom CSS code to address these display problems.

[timelsass]

This is dependent on quite a few other changes that have to happen, but typography is something that needs to be changed. I don't think we will add controls for different viewport sizes in the theme though, but it is a possibility.

Just as an initial idea, I originally thought of doing something along the lines of this pen:
https://codepen.io/times/pen/qGWXer

That's using the typography scale defined for heading sizes(bootstrap), but I'd like to introduce some "scale" controls which would change the typography rhythm used. As an example of this, look at the site modularscale.com.

Overall, I'd like to see a solution taking the typography scales mentioned above, and then setting some minimum and maximum thresholds for the viewpoints. We can then use the linear interpolation technique shown in the pen above to allow for sizing the relationships between viewports automatically, all based on the user scaling the content size fluidly. Once this is implemented it will be trivial to add controls in for a custom scale, so everything can be controlled in a more visual way.

There's more to it all, but ideally I want to remove the bootstrap dependency we currently have first. There's been a large amount of work to getting to that point put forth so far, but typography is one of the bigger barriers left.

[jessecowens]

@jamesros161 this feature has been requested by another user in the BoldGrid Support Forums.