v1.7.8 release

Deprecate Node.Text

Node.Text was intended to get a text value from some inline nodes.
A 'text value' of a Text node is clear.

But

- BaseNode had a default implementation of Node.Text
- Lacks of GoDoc description that Node.Text is valid only for
  some inline nodes

So, some users are using Node.Text for BlockNodes.

A 'text value' for a BlockNode is not clear.

e.g. : Text value of a ListNode

- It should be contains list markers?
- What do characters concatinate List items with? newlines? spaces?
- If it contains codeblocks, codeblocks should be fenced or indented?

Now we would like to avoid such ambiguous method.