Improve pgloader usage from the command line.

Make it so that the following command line usages are accepted when
using pgloader without a command file:

 ./build/bin/pgloader ./test/sqlite/sqlite.db postgresql:///pgloader

 ./build/bin/pgloader --set "search_path='sakila'"  \
                      mysql://root@localhost/sakila \
                 postgresql:///sakila

 ./build/bin/pgloader --type csv                             \
                      --field id --field field               \
                      --with truncate                        \
                      --with "fields terminated by ','"      \
                      ./test/data/matching-1.csv             \
                      postgres:///pgloader?matching

It's now possible in most cases to just use command-line options, which
should make the entry bar to pgloader much lower.