sortMethod simplification.

[Michael-Gallo]

The iota enum expression for sortMethod required the same lengthy switch statement in setExpression's eval and setLocalExpression's eval, along with getOptsMap in main to be updated for every sort method. Making sortMethod's underlying representation a string removes the need to have these long and repetitive switch statements and for new sortMethods to require changes outside of dir.sort in nav.go and the const definitions at the top of opts.go

Alternatively, sortMethod can stay as an iota and there can be a map of sortMethods to their string representations.

[joelim-work]

Thanks for this patch. From what I can see in the commit history, it looks like from a0e1d4d the sort method actually used to be a string. I'm not sure the reason behind the change though, so I will leave it to @gokcehan to decide before merging.

[gokcehan]

@joelim-work I think my rationale for that commit was to have a single line comparison if d.sortType != gOpts.sortType { ... }, but I don't see any reason why we can't simply use strings. As a further cleanup, I'm also in favor of simply getting rid of the sortType struct and having everything under gOpts since the single line comparison is already outdated in this case. Anyway, thank you @Michael-Gallo for the patch and @joelim-work for the review.