Improve concards syntax for flip sides.

[alanxoc3]

When you have "sides" to a flashcard, normally, you only have two sides. The colon ":" syntax allows the parent to be reversed to the front side. Right now there can be one front side and multiple back sides, but what if there were multiple "front" sides? There would need to be some sort of syntax improvement for this.

[alanxoc3]

I figured this out. See #43. I will have a :: syntax instead of the current : syntax. This new syntax will also be more powerful than the current : syntax. It will take precedence over the |, which will allow for cards like this:

This flashcard section:
:> 你好世界 | 嘿世界
:: Hello World | Greetings World
:: Saluton al la mundo
<:

Will generate all these:
:> 你好世界 | Hello World | Greetings World | Saluton al la mundo
:> 嘿世界 | Hello World | Greetings World | Saluton al la mundo
:> Hello World | 你好世界 | 嘿世界 | Saluton al la mundo
:> Greetings World | 你好世界 | 嘿世界 | Saluton al la mundo
:> Saluton al la mundo | 你好世界 | 嘿世界 | Greetings World
<:

[alanxoc3]

Has been merged into master already.