feat: add charset flag

[mavogel]

What does this do / why do we need it?

Some files need to stay with specific charset, so I used paulrosania/go-charset to convert charset of environment variables to replace in documents when use -c flag.

What should your reviewer look out for in this PR?

are there tests?

Check lists

• [ ] Test passed
• [ ] Coding style (indentation, etc)

Which issue(s) does this PR fix?

• see #29