Allow headers to be passed to the seeder application

[geographika]

Similar to #258 - this pull request allows custom headers to be passed to the seeder application, in order to forward these to the back-end server.

The same approach was taken as with the -D (dimensions) command line paramter. The code to parse KEY=VAL was extracted to its own function so it could be reused with a new -H header switch.

Both dimension and header tables are initialized with 3 items:

argdimensions = apr_table_make(ctx.pool,3);
argheaders = apr_table_make(ctx.pool, 3);

I ran the seeder with 4 -d swtiches and it didn't seem to cause an issue. Is the table size automatically increased?

If accepted then the seeder docs page will also be updated. Note the -b switch also need to be documented (pull request).