Use diff or similar tool to compare snapshots

[lukfor]

This PR improves comparing huge and complex snapshots. (closes #67)

nf-test checks if diff is installed on the system and use it to compare the snapshots side by side. The arguments can be changed with the environment variable NFT_DIFF_ARGS (default arguments: -y -W 200).

To change to a different tool, the env variable NFT_DIFF can be used.

Example to use icdiff (it supports colors etc..):

export NFT_DIFF="icdiff"
export NFT_DIFF_ARGS="-N --cols 200 -L expected -L observed -t"

Output of icdiff:

Open tasks:

• test on ubuntu
• add to documentation