Export all .env variables declared at the start of a line.

This change is in response to #28. After the .env is sourced, sed is run to extract the names of all declared variables, and these are exported. Exporting is required in order to use the variables in child processes and to remain compatible with Foreman.

Variable declarations that include whitespace at the beginning of the line aren't supported because Foreman and other utilities that parse .env files don't support them.