cheapcmd.vim enable command-line-completion to expand short keyword of user-defined commands.
The short keyword consists of any alphabets appended colon(:) or semicolon(;) to end.

e.g.)

• vhg; -> VimHelpGenerator, VimHelpGeneratorVirtual
• vhgv; -> VimHelpGeneratorVirtual
• qr; -> QuickRun
• cp: -> CtrlP, CtrlPBuffer, CtrlPClearAllCaches, ... cpfile, cprevious
• cpb: -> CtrlPBuffer
• cb; -> ConflictMarkerBoth, CtrlPBuffer

The word started lower character and contained upper character or
the word contained hyphen is keyword, too.

• unR -> UniteRecordingBegin, UniteRecordingSave, UniteResume
• unRes -> UniteResume
• un-rec -> UniteRecordingBegin, UniteRecordingSave

"for cmdline
cmap <Tab> <Plug>(cheapcmd-expand)

"for cmdwin
aug cheapcmd-cmdwin
  autocmd!
  autocmd CmdwinEnter * call s:define_cmdwin_mappings()
aug END
function! s:define_cmdwin_mappings()
  nmap <buffer><Tab> <Plug>(cheapcmd-expand)
  imap <buffer><Tab> <Plug>(cheapcmd-expand)
endfunction

Type <Tab> after short keyword of user-defined commands in command line head.