Refactor and add option to choose clipboard program

[andy941]

Proposed change

Allow the user to specify which clipboard program to use when pasting images from the clipboard.

Type of change

I refactored the code a little bit by putting all the clipboard commands in a lookup table. I also changed the docs.

P.S.: this is my first time making a contribution to a plugin and I am not familiar with lua so let me know if there are any issues. I tested a bunch of cases on my end and it seems to work well. On my machine the xsel executable produces empty files while xclip works like a charm.

• [ ] Bugfix (non-breaking change which fixes an issue)
• [ x] New feature (thank you!)
• [ ] Code quality improvements to existing code or addition of tests
• [ x] Documentation update

Additional information

• This PR fixes or closes issue: fixes #320
• This PR is related to issue: #320

Checklist

• [x] I am running the latest version of the plugin.
• [x] The code change is tested and works locally.
• [x] There is no commented out code in this PR.
• [x] The code has been formatted using Stylua (a .stylua.toml file is provided)
• [x] The code has been checked with luacheck (a .luacheckrc file is provided)
• [ ] The README.md has been updated according to this change. // not relevant?
• [x] The doc/telekasten.txt helpfile has been updated according to this change.

[lambtho12]

Thank you for the PR!

I am at work now. I will try to have a look at it this evening or tomorrow and merge it asap.

Feel free to tackle any other small issue if you want to get more involved with Telekasten. We desperately need all the help we could get!

[lambtho12]

Thank you very much for your contribution! We are always looking for new contributors to the project. If you have time, do not hesitate to tackle other pending issues.