Allow passing custom flags to db exec

[shadyvb]

Some use cases might need passing additional flags to mysql, eg: -N or --skip-column-headers to suppress column headers, which is currently not possible.

eg:

composer server db exec -- -N 'select option_value from wp_options where option_name = \'siteurl\';'

Should be possible with a simple change, will post a PR shortly, just creating an issue for visibility.