expose field object publicly

[ashtonian]

This adds functions to expose the internal fields object and to create a new parser from a slice of field objects. This lets the user dynamically configure a parser, my use case is to change rql config based on permissions. I would also like to expose the parsed data and related field information to be able to document related metadata for the user like what are available operations. I think it pairs well with #51.

I think it would be nice to refactor to the following, but this is breaking:

func NewParser(c Config, Model interface{}){}
func NewParserF(c Config,  fields []*Fields){}

Added:

func NewParserF(c Config, fields []*Field) (*Parser, error) {}
func (p *Parser) GetFields() []*Field {}

[a8m]

Why not inline its fields inside the Field type?

[a8m]

Suggested change

	func (p *Parser) GetFields() []*Field {
	fields := make([]*Field, 0, len(p.fields))
	for _, v := range p.fields {
	fields = append(fields, v)
	}
	return fields
	}
	func (p *Parser) Fields() []*Field {
	fields := make([]*Field, 0, len(p.fields))
	for _, v := range p.fields {
	fields = append(fields, v)
	}
	return fields
	}

Why this method is needed? If it's for testing, let's use the existing public API instead.

[a8m]

Rename to: NewFieldsParser or add a new option as Config.Fields.

Also, an error should return if c.Model was set as the two are mutually exclusive.

[a8m]

Thanks for the contribution, @ashtonian. It's appreciated.