Add an unalias command

Add bin/unalias.bat to perform the opposite action of alias.bat.
Removes a given alias from the current shell environment and the
aliases file.

Fixes issue #281.

bin/unalias.bat

@@ -0,0 +1,28 @@
+@echo off
+if ["%1"] == ["/?"] goto:p_help
+if ["%1"] == [""] echo Insufficient parameters. & goto:p_help
+
+setlocal
+
+:: Check if alias exists
+doskey /macros | findstr /b %1= >NUL || goto :p_not_found
+
+:: Remove alias from current shell
+doskey %1=
+
+:: Remove alias from aliases file
+copy /y "%CMDER_ROOT%\config\aliases" "%TEMP%\aliases.prev" >NUL
+type "%TEMP%\aliases.prev" | findstr /b /v %1= > "%CMDER_ROOT%\config\aliases"
+echo Alias removed
+
+endlocal
+goto:eof
+
+:p_not_found
+echo Alias not defined.
+goto:eof
+
+:p_help
+echo.Usage:
+echo.	unalias name
+echo.	For more information, read DOSKEY/?