In systems that use the MAWK interpreter (the default AWK installed with
Ubuntu), the output of `rbenv help <command>` would have no line breaks.
The issue is fixed by changing `gsub` to `sub` in the snippet of awk
commands that are used to extract documentation comments.
I suspect the bug is something to do with the way the '^' and '$'
characters are interpreted by different AWK interpreters (per-line vs
per-string anchors).
If I understand correctly, the purpose of trim() is to remove all line
breaks from the start and end of each sections of a command's
documentation, in which case `sub` should serve the same purpose.
A command doesn't have to specify Usage docs if it doesn't accept any
arguments. The default usage for a command will be printed as:
Usage: rbenv ${command}
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
