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

internal/p10k.zsh

@@ -3785,6 +3785,30 @@ _p9k_init_params() {
   #
   # These correspond to `java -fullversion` and `java -version` respectively.
   _p9k_declare -b POWERLEVEL9K_JAVA_VERSION_FULL 1
+
+  local -i i=1
+  while (( i <= $#_POWERLEVEL9K_LEFT_PROMPT_ELEMENTS )); do
+    local segment=${(U)_POWERLEVEL9K_LEFT_PROMPT_ELEMENTS[i]}
+    local var=POWERLEVEL9K_${segment}_LEFT_DISABLED
+    (( $+parameters[$var] )) || var=POWERLEVEL9K_${segment}_DISABLED
+    if [[ ${(P)var} == true ]]; then
+      _POWERLEVEL9K_LEFT_PROMPT_ELEMENTS[i,i]=()
+    else
+      (( ++i ))
+    fi
+  done
+
+  local -i i=1
+  while (( i <= $#_POWERLEVEL9K_RIGHT_PROMPT_ELEMENTS )); do
+    local segment=${(U)_POWERLEVEL9K_RIGHT_PROMPT_ELEMENTS[i]}
+    local var=POWERLEVEL9K_${segment}_RIGHT_DISABLED
+    (( $+parameters[$var] )) || var=POWERLEVEL9K_${segment}_DISABLED
+    if [[ ${(P)var} == true ]]; then
+      _POWERLEVEL9K_RIGHT_PROMPT_ELEMENTS[i,i]=()
+    else
+      (( ++i ))
+    fi
+  done
 }
 
 typeset -ga __p9k_wrapped_zle_widgets