Add plugin support for left prompt (PROMPT)

[desyncr]

• PROMPT now supports plugin registration with the same mechanism as RPROMPT.
• Added path and hostname plugins for PROMPT

PENDING

• [x] Update README
• [x] Add configuration examples

From: https://trello.com/c/U2xwxwzJ/13-enable-left-prompt-customization

Configuration examples

Default

GEOMETRY_PROMPT_PLUGINS_PRIMARY=(path hostname)
GEOMETRY_PROMPT_PLUGINS_SECONDARY=(exec_time jobs git hg)

Full

GEOMETRY_PROMPT_PLUGINS_PRIMARY=(path hostname)
GEOMETRY_PROMPT_PLUGINS_SECONDARY=(exec_time jobs node ruby rustup virtualenv docker_machine git hg)

Custom prompt

GEOMETRY_PROMPT_SUFFIX=$'\n └─▪'
GEOMETRY_PROMPT='┌─ [$(whoami)]'
GEOMETRY_PROMPT_PLUGINS_PRIMARY=(path hostname)

Movable plugins

GEOMETRY_PROMPT_PLUGINS_PRIMARY=(path node)
GEOMETRY_PROMPT_PLUGINS_SECONDARY=(git)

[desyncr]

This is done. Let me know what you guys think!

[desyncr]

@fribmendes @jedahan Do we merge it?

[frm]

@desyncr it's a go from my end.