Allow configuration options to be read in from and written to file

[jimc101]

Options for ecoval are passed in as optional keyword arguments. If the option isn't passed in, ecoval will (in many cases) select a default value. Run time options are not recorded (I think), meaning there is no way of checking which options were used for a given analysis.

It would be useful to have ecoval read in run arguments from a file, as well as passing them in as arguments should a user want to. Further, it would be useful to have ecoval write out all of its run time options to a file which is saved with the output. This includes default values - i.e., those that weren't specified by the user. configparser would allow you to do both of these things.