nvm non-blocking sourcing

.bashrc

# Replace following lines
export NVM_DIR="/home/USERNAME/.nvm"
[ -s "$NVM_DIR/nvm.sh" ] && . "$NVM_DIR/nvm.sh"  # This loads nvm

init_nvm()
{
    bin=$(echo $1 | awk '{print $1}') # get first word from command string
    if [[ -z $NVM_DIR ]] && [[ ! `command -v $bin` ]]; then # if not already inited and no such binary found
        export NVM_DIR="/home/USERNAME/.nvm"
        [ -s "$NVM_DIR/nvm.sh" ] && . "$NVM_DIR/nvm.sh"  # This loads nvm
    fi
}
preexec_functions+=(init_nvm)