allow prompt segments to be disabled through configuration options

Prompt segment `foo` doesn't get evaluated or shown on the left
if `${POWERLEVEL9K_FOO_LEFT_DISABLED-$POWERLEVEL9K_FOO_DISABLED}`
is equal to `true`.

Example:

    POWERLEVEL9K_LEFT_PROMPT_ELEMENTS=(dir)
    POWERLEVEL9K_RIGHT_PROMPT_ELEMENTS=(dir)

  - By default `dir` is shown on the left and on the right.
  - `POWERLEVEL9K_DIR_LEFT_DISABLED=true` disables `dir` on the left.
  - `POWERLEVEL9K_DIR_RIGHT_DISABLED=true` disables `dir` on the right.
  - `POWERLEVEL9K_DIR_DISABLED=true` disables `dir` everywhere.

Note that it's possible to hide segments without disabling their
evaluation through expansions. For example:

  POWERLEVEL9K_DIR_CONTENT_EXPANSION='${HIDE_DIR-$P9K_CONTENT}'
  POWERLEVEL9K_DIR_VISUAL_IDENTIFIER_EXPANSION='${HIDE_DIR-$P9K_VISUAL_IDENTIFIER}'

With these settings, `HIDE_DIR=''` hides `dir` while `unset HIDE_DIR`
restores it.

fixes #205

0 files changed, 0 insertions, 0 deletions