restore tab completion for commands. closes #1172
group 'simple-prompt' toggle with other prompt commands in `help`.
write consistent casing in `help` command.
support deprecated method for HEAD support with pry-doc.

closes #1152.
related to #1136 but `? File.exists?` is still not showing me documentation.
thanks to @yui-knk for finding the bug & providing a solution.
update `help` test.
update `toggle-color` to go through _pry_.color.
update CHANGELOG.md
fix inconsistency in wording between list-prompts and list-inspectors
add local names from `Pry#sticky_locals` to list of custom input completions.