For just a handful of people, rbenv init would hang indefinitely. Turning on
debugging output suggested that the `read` expression to split PATH into a bash
array was hanging, but I could never reproduce this myself. Instead, this uses
bash v4+ `readarray` if it's available, or falls back to bash v3 `read` with the
default DELIM being a newline character. This will cause a regression if any PATH
entries contain a literal newline character, but hopefully people are not relying
on such paths.
Docs are comprised from "Usage", "Summary" and "Help" sections, where
"Help" can span multiple commented lines. If it is missing, "Summary" is
shown in its place.
References #204, references #206
