add POWERLEVEL9K_<segment>_DISABLED_DIR_PATTERN

This parameter allows you to disable a given prompt segment in
any directory that matches the specified pattern.

For example, if you have a Git repository in /foo/bar for which
you don't want to see Git status in prompt, define the following
parameter:

    POWERLEVEL9K_VCS_DISABLED_DIR_PATTERN='/foo/bar(|/*)'

See #300.

internal/p10k.zsh

@@ -3432,6 +3432,8 @@ function _p9k_set_iface() {
 
 function _p9k_build_segment() {
   _p9k_segment_name=${_p9k_segment_name%_joined}
+  local disabled=POWERLEVEL9K_${(U)_p9k_segment_name}_DISABLED_DIR_PATTERN
+  [[ $_p9k_pwd == ${(P)~disabled} ]] && return
   if [[ $_p9k_segment_name == custom_* ]]; then
     _p9k_custom_prompt $_p9k_segment_name[8,-1]
   elif (( $+functions[prompt_$_p9k_segment_name] )); then
@@ -3442,6 +3444,8 @@ function _p9k_build_segment() {
 
 function _p9k_build_instant_segment() {
   _p9k_segment_name=${_p9k_segment_name%_joined}
+  local disabled=POWERLEVEL9K_${(U)_p9k_segment_name}_DISABLED_DIR_PATTERN
+  [[ $_p9k_pwd == ${(P)~disabled} ]] && return
   if (( $+functions[instant_prompt_$_p9k_segment_name] )); then
     local -i len=$#_p9k__prompt
     _p9k_non_hermetic_expansion=0