Add support for sup and sub html tags

[cgreening]

We'll need to check how much room we have for more fonts - I'm wondering if the epdiy library could be modified to support scaling of existing fonts. That might give us an easy way of drawing fonts at half size for the super and subscript text.

[martinberlin]

There is also statically allocated memory (both IRAM and DRAM), you can see the amount by running "make size". Plus a 64kB region of IRAM used for CPU caches.

There was a command in esp-idf that let's you know exactly how much Data RAM/ Instruction RAM the firmware is using.
This article about optimizing IRAM usage is worth a good read.