Quick reference until or unless mcrcon adds support for this behavior automatically
MCRCON_PASS="$(sed -n -e 's/^rcon\.password=\(.\+\)/\1/p' < server.properties)" mcrcon
Alternatively, a bash alias:
alias mcrcon='env MCRCON_PASS="${MCRCON_PASS:=$(sed -n -e '\''s/^rcon\.password=\(.\+\)/\1/p'\'' < server.properties)}" mcrcon'