[#13] Each command function requires the root path of an Inkt directory.
[#13] "Preview" command launches a server.
[#13] Error messages are printed by "PrintError" function.
[#13] Fix: "PrintError" requires a value satisfying "io.Writer".
[#13] Refactor "Config".
[#13] Use "http.ServeFile".
[#13] Remove a commented-out needless line.
[#13] Refactor "Config".
Merge branch 'feature/preview-server' into develop