Feature/28: add unit tests for some placeholders and refactor conversion code

[EmilyGraceSeville7cf]

closes #28

ToDo

• [x] device placeholders conversion fixes and unit tests
• [x] user placeholders conversion fixes and unit tests
• [x] group placeholders conversion fixes and unit tests
• [x] ip placeholders conversion fixes and unit tests
• [x] database placeholders conversion fixes and unit tests
• [x] argument placeholders conversion fixes and unit tests
• [x] option placeholders conversion fixes and unit tests
• [x] setting placeholders conversion fixes and unit tests
• [x] subcommand placeholders conversion fixes and unit tests
• [x] extension placeholders conversion fixes and unit tests
• [x] file_or_directory placeholders conversion fixes and unit tests

These placeholders don't have unit tests now, but they will be added soon:

• [x] file placeholders conversion fixes Note: handle path/to/file.tar[.ext|.ext2|...] specially: convert to path/to/file.ext
• [x] directory placeholders conversion fixes
• [x] boolean placeholders conversion fixes
• [x] integer placeholders conversion fixes
• [x] float placeholders conversion fixes
• [x] string placeholders conversion fixes
• [x] char placeholders conversion fixes

Notes

• number suffixes like {{integer4}} (4 is suffix) are preserved just for singular placeholders, for plural they are ignored while conversion

Found bugs

• {{files{1,2,3}}} doesn't expand to {file* value}

[EmilyGraceSeville7cf]

@kbdharun, @mfrw, can u test converter and merge PR after it if everything is okay?

[EmilyGraceSeville7cf]

I am going to merge it, it seems to work fine. Already tested it while converting TlDr pages.