feature request: Add ability for extra fields(i.e. problem id)

[dayo05]

Lots of plugins including AutoCP supports file creation based on its name or some contents.

For Instance, Example on README file name will be Castle_Defense.cpp or similar.

Some OJ uses problem ID for problem identification. Particularly, Beakjoon Online Judge uses 4~5 digit number for identification. Codeforces has [ContentNumber][ProblemId], like 954G.

All of OJ has different format for it. So I suggest to give ability of custom, optional field which can be specific to OJ.(Like problem number in Beakjoon Online Judge)