move REPL implementation to separate class
add Boris as a dependency and use it instead of homegrown REPL
fall back to old REPL implementation if Boris is not available
add --basic flag to `wp shell` command
remove support for $_ special variable, since Boris doesn't support it.
fix shell tests
shell: use var_dump(), to better match Boris output
shell: remove support for 'history' builtin

1. The syntax felt kind of inconsistent.
2. Boris doesn't support it.
3. It didn't ship with a stable release, so not many people know about
it.

see #373
shell man: remove note about backslashes

Backslashes are needed only in basic mode.
shell: remove Boris tests

they belong in the Boris repo
make Boris a suggested package
shell: remove superfluous exit instructions
travis: force-install Boris, for smoke testing
shell: re-generate man page

[ci skip]
don't install Boris in dev-build, since it alters the git repo
Merge pull request #437 from wp-cli/boris

Use Boris to power `wp shell`

fixes #292 and #316