Add custom style option directive

[jays1204]

User can add custom style option of carousel list element. rn-carousel-custom-style direction has json data. This direction object can have four keys.

• all : apply style with every carousel list element
• first : apply style when first list element visible
• middle : apply style when second ~ before last carousel list element visible
• end : apply style when last list element visible

Above four key have css style key & value. and also special key with prefix "_rest-". This style value of key apply with non visible carousel list element.