More features for TextLineWrapper.

More features for TextLineWrapper.

Add more control over formatting the wrapped lines:
- Add possibility to indent the wrapped lines.
- Add possibility to indent the first line after an explicit line break
  differently from other lines.
- Add possibility to mark line continuation using an explicit character.
- Add possibility to either remove or keep leading whitespace after an
  explicit line break.
All except the last are already useful for the current console output.
Depending on the chosen solution, some or all of these are also needed
for #969.

Also expose the lower-level interface used internally that makes it
possible to write code that iterates over the wrapped lines and does
something else than just format them into a buffer.  May be useful for
some cases in #969, but is necessary to use the wrapper for #838 (line
wrapping in error messages).

Change-Id: I905ba29856773656bf000c4b2e14d1ed2ba4de31