Talisman fails silently if parts of the .talismanrc are not valid yaml

[karlbrown-va]

Describe the bug
.talismanrc is supposed to be a yaml file. However, the filename does not end in .yaml, certain editors may not detect it and use yaml settings, and additionally, yaml linters will not detect it by default to find errors.

This can lead to silent failures - for example if a tab is added to one of the lines. Further lines in the file (such as custom_patterns) could be ignored, leading to password leakage.

To Reproduce
Steps to reproduce the behavior:

1. Create a new git repository and add a test file
2. Add a custom pattern to .talismanrc:

custom_patterns:
  - 'pwd_[a-z]{8,20}'

4. Add a string matching this pattern to the file
5. git add . and then talisman -l debug --githook pre-commit and confirm the secret is detected and the custom pattern was used
6. Replace the spaces in .talismanrc with tabs
7. git add . and then talisman -l debug --githook pre-commit and confirm the secret no longer detected

Expected behavior

• Talisman should fail on a malformed .talismanrc.yaml file and perform some basic sanity checks on the file
• .talismanrc should be renamed as .talismanrc.yaml so it is detected by yamllinters and editors.
• The samples of .talismanrc.yaml should be updated to be proper yaml (with correct indenting)
• The output of the checksum tool should be properly indented (currently it is not indented, missing --- prefix, - e.g.

fileignoreconfig:
- filename: readme.md
  checksum: xxx
version: ""

should be

---
fileignoreconfig:
  - filename: readme.md
    checksum: xxx
version: ''

Desktop (please complete the following information):

• OS: OSX

[tw-owen-nelson]

After #458 and #461 talisman will fail on a malformed .talismanrc file (i.e. if it cannot unmarshal the content into our expected struct).

I'm open to checking for a .talismanrc.yaml file in addition to .talismanrc.

Indenting and prefixing have less clear value to me. Is this so that the generated .talismanrc[.yaml] complies with yaml linters? We currently leave this formatting to go-yaml/yaml's marshaller and would need to reformat ourselves to achieve that.

[karlbrown-va]

Yes, I think if it's yaml it should comply with the linter. Would be great if you accepted .yaml / .yml files (both prefixes seem to be allowed in the yaml world)