First patch -- somewhat trivial feature

This is my first attempt at writing a patch, so it's pretty simple.

Commit log sums it up:

Adds a convenience feature to the explain command which prints out GUC cost
parameters in explain text output.

For example:

explain (params) select * from table;

will include text output like the following:

Cost Params:
seq_page: 1.000000, rnd_page: 4.000000, cpu_tup: 0.010000,
cpu_ind: 0.005000, cpu_op: 0.002500
amenabled: 11111111111

The bit vector is enable variables in the order listed in cost.h,
though mainly provides a high level view on whether or not any strategies
are disabled.

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I recognize that this is kind of a frivolous feature which may not be worth
any potential maintenance burden, so submitted for what it's worth as an
initial effort.

Best Regards,
Robert