StringToSlice now trims white space on each item in the slice

[thrawn01]

Purpose

Allow users to trim and modify slice items during parse. Addresses #39

Implementation

// Returns []string{"one"}
result := args.StringToSlice("one")

// Returns []string{"one", "two", "three"}
result := args.StringToSlice("one, two, three", strings.TrimSpace)

// Returns []string{"ONE", "TWO", "THREE"}
result := args.StringToSlice("one, two, three", strings.ToUpper, strings.TrimSpace)

[coveralls]

Coverage increased (+0.1%) to 67.878% when pulling ffe2f2c on trim-whitespace-StringToSlice into d582419 on master.

[coveralls]

Coverage increased (+0.1%) to 67.878% when pulling a922f25 on trim-whitespace-StringToSlice into d582419 on master.

[coveralls]

Coverage increased (+0.03%) to 67.782% when pulling 5a4d17f on trim-whitespace-StringToSlice into d582419 on master.