Add the ability to pull configuration data from the environment.

[erikogan]

(First off, let me say thank you for developing ag. It has made searching our large codebase(s) a snap!)

This PR adds environment variables for many of the command-line options.

I also added a command-line option (and environment variable) to ignore the environment for configuration.

In the case of a boolean configuration option, environment variables set without a value currently do not affect the value, but it would be easy enough to change that. (I am currently embroiled in a long argument with myself about how they should be handled.)

I am open to any comments you might have but especially the names of the functions and environment variables.

[Along the way, I noticed that opts.before & opts.after are both size_t, whereas opts.context is an int. I don’t think there is any value to a negative context, and I may open a separate issue/PR for that, since context is the only reason for the env_int_option function.]