Command history truncates multiline commands

• iTerm2 version: Build 2.9.20160229-nightly
• OS version: 10.11.3 (15D21)

Multiline commands are truncated in the history to the first line, e.g. running:

if true; then echo hello fi

Results in just if true; then in the history. Similarly when splitting long commands by escaping the newlines with a backslash.

There are a lot of issues here:

1. In zsh and bash, PS2 is configurable. Detecting it is nontrivial. We could customize it.
2. In zsh and tcsh (but not bash), precmd runs at the end of the first line of a multiline command, not after the last continuation prompt. We'll need to customize PS2 on zsh to work around this. Not sure what to do about tcsh.

Fish should basically just work if we keep the newlines.

Milestone changed to Feature Complete 3.1

Milestone changed to Feature Complete 3.2