v0.2.1 - ruthless link stripper

This is a tidy up for v0.2 which ensures that links are only preserved if the block was defined with allow_links=True. For example:

from wagtail.blocks import StreamBlock
from wagtail_tinytableblock.blocks import TinyTableBlock

class ContentBlocks(StreamBlock):
    table_block = TinyTableBlock(allow_links=True)

It also improves the input sanitization