Extend list command by additional output formats

[lukfor]

This PR extends the list command by:

• --tags to get a list of all tags
• --format json to get the output as json
• --format raw to get tests without formatting
• --silent to hide the program header

Open tasks:

• Copy examples to docs and describe flags/options

Examples:

List of all testcases as json:

nf-test list --format json --silent
["/Users/lukfor/Development/git/nf-gwas/tests/main.nf.test@69b98c67","/Users/lukfor/Development/git/nf-gwas/tests/main.nf.test@fdb6c1cc","/Users/lukfor/Development/git/nf-gwas/tests/main.nf.test@d1c219eb","/Users/lukfor/Development/git/nf-gwas/tests/main.nf.test@3c54e3cb",...]

List of all testcases as unformatted ist:

nf-test list --format raw --silent
/Users/lukfor/Development/git/nf-gwas/tests/main.nf.test@69b98c67
/Users/lukfor/Development/git/nf-gwas/tests/main.nf.test@fdb6c1cc
/Users/lukfor/Development/git/nf-gwas/tests/main.nf.test@d1c219eb
/Users/lukfor/Development/git/nf-gwas/tests/main.nf.test@3c54e3cb
...

List of all tags as json:

nf-test list --tags --format json --silent
["fastqc","snakemake"]

List of all tags as unformatted list:

nf-test list --tags --format raw --silent
fastqc
snakemake

[edmundmiller]

Is there a quick dev env I can hop into to test this or a script I can download?

The code looks good to me!