ini: do not outout back quotes if IgnoreInlineComment enabled (go-ini#99

)

parser: remove surrounding quotes for empty value (go-ini#97)

section: add ChildSections (go-ini#95)

Also added (*File).ChildSecions.

parser: add LoadOptions.IgnoreInlineComment (go-ini#93)

struct: add more map to types with shadow (go-ini#92)

struct: support map to []int and []int64 (go-ini#92)

struct: support shadows for MapTo (go-ini#92)

Only works for []string type field.

Add new function ShadowLoad

key: able to write out shadows

key: add shadow keys