plugin/man: simplify

Don't overwrite variables that the user has already set.

Author: gaelicWizard

plugins/available/man.plugin.bash

@@ -1,12 +1,14 @@
 cite about-plugin
 about-plugin 'colorize man pages for better readability'
 
-export LESS_TERMCAP_mb=$'\e[1;32m'
-export LESS_TERMCAP_md=$'\e[1;32m'
-export LESS_TERMCAP_me=$'\e[0m'
-export LESS_TERMCAP_se=$'\e[0m'
-export LESS_TERMCAP_so=$'\e[01;33m'
-export LESS_TERMCAP_ue=$'\e[0m'
-export LESS_TERMCAP_us=$'\e[1;4;31m'
+: "${LESS_TERMCAP_mb:=$'\e[1;32m'}"
+: "${LESS_TERMCAP_md:=$'\e[1;32m'}"
+: "${LESS_TERMCAP_me:=$'\e[0m'}"
+: "${LESS_TERMCAP_se:=$'\e[0m'}"
+: "${LESS_TERMCAP_so:=$'\e[01;33m'}"
+: "${LESS_TERMCAP_ue:=$'\e[0m'}"
+: "${LESS_TERMCAP_us:=$'\e[1;4;31m'}"
+
+export "${!LESS_TERMCAP@}"
 
 export LESS="R${LESS#-}"