This commit introduces 4 macros (for different log levels) that can be used
to log messages to $HOME/.nvimlog:
- DLOG: log a debug message (e.g. `DLOG("sum(%d, %d): %d", x, y, sum(x, y));`)
- ILOG: log some useful information (e.g. `ILOG("Main loop started")`)
- WLOG: log a warning (e.g. `WLOG("Command not found: %s", command)`)
- ELOG: log an error (e.g. `ELOG("Out of memory. Exiting.")`)
All these macros are disabled if `NDEBUG` or `DISABLE_LOG` is defined. This
guarantees that a `Release` build won't log anything.
`MIN_LOG_LEVEL` can be defined to reduce the verbosity of the log. The log
levels are:
```
DEBUG_LOG_LEVEL 0
INFO_LOG_LEVEL 1
WARNING_LOG_LEVEL 2
ERROR_LOG_LEVEL 3
```
`MIN_LOG_LEVEL` is 0 by default enabling all levels. If `MIN_LOG_LEVEL` is set
to 2, for example, only warnings and errors will be logged.
That's how the log looks like:
```
DATETIME LOG_LEVEL FUNCTION LINE PID FORMATTED MESSAGE
2014/05/01 23:46:14 [info @ main_loop:582] 44376 - Starting Neovim main loop.
2014/05/01 23:46:31 [info @ main_loop:582] 44400 - Starting Neovim main loop.
```
