output.txt

$ seddiff 's/redcarpet/magiccarpet/;s/pygments/rouge/' _config.yml
Result of s/redcarpet/magiccarpet/;s/pygments/rouge/ against _config.yml:
7,8c7,8
< markdown:         magiccarpet
< highlighter:      rouge
---
> markdown:         redcarpet
> highlighter:      pygments
54c54
<   - magiccarpet
---
>   - redcarpet
56c56
<   - rouge.rb
---
>   - pygments.rb
71c71
<   - magiccarpet
---
>   - redcarpet
73c73
<   - rouge.rb
---
>   - pygments.rb

seddiff.sh

function seddiff () { sed -i.bak "$1" $2; echo "Result of $1 against $2:"; diff ${2}{,.bak}; }