Use asciiDoc instead of markdown

[thomaseizinger]

Use ascii doc for all templates so that it is default for future spikes and rfcs.
Also, unify spike templates.

• https://github.com/comit-network/spikes/
• https://github.com/comit-network/rfcs/
• https://github.com/coblox/spikes/

[bonomat]

I like it. Just tried it out in comit-network/spikes#6.

[LLFourn]

Closed wtth #67