Implement custom replacer upload

[ancientjpeg]

We'd like the users to be able to pass a custom replacer. The following formats should be accepted:

• Textbox Editor (use YAML/plaintext)
• YAML/plaintext — just simple k/v pairs separated by colons and newlines
• JSON

[ancientjpeg]

Please note that YAML was abandoned, and file upload is now added to issues as #29.