Support string and number literal types

[motiz88]

This fixes breakage I've seen when trying to generate HTML documentation from Flow types that contain literal types as their members, (e.g. member types of disjoint unions). String and number literals are both supported to match Flow. (Possibly booleans are also needed? The Flow changelog hints at this)

I couldn't figure out how to add proper tests, unfortunately, but the minimal case that used to cause errors is running documentation build --format html on the following two variations of the same code:

/**
 * A
 */
type A = {tag: 'A';};

/**
 * A
 */
type A = {tag: 1;};