start-doltlab.sh should use .env

The start doltab shell script should use .env files by default if found so that server admins only have to set these once and run the script instead of constantly setting these env vars.

I suggest something like the following code:

```
unamestr=$(uname)
if [ -f .env ]; then
echo 'environment file found'
if [ "$unamestr" = 'Linux' ]; then
export $(grep -v '^#' .env | xargs -d '\n')
elif [ "$unamestr" = 'Darwin' ]; then
export $(grep -v '^#' .env | xargs -0)
fi
fi
```

this would load the .env into the environment at runtime