Add r (read file) command

[erikgaas]

Adds the classic ex r command to exhash — reads a file's contents and inserts them after the addressed line.

Changes

• parse.rs: New Subcommand::Read { path: String } variant, parses r filepath after lnhash address. Also allows 0|0000| with r (same as a/i).
• engine.rs: New read_file method that reads the file, splits into lines, and reuses append_after to insert them.

Example

# Insert contents of other.txt after line 2
2|f7c4|r other.txt