Macro-based log utility for Neovim

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.
```

src/main.c

@@ -33,6 +33,7 @@
 #include "misc2.h"
 #include "crypt.h"
 #include "garray.h"
+#include "log.h"
 #include "memory.h"
 #include "move.h"
 #include "normal.h"
@@ -578,6 +579,7 @@ main_loop (
   linenr_T conceal_new_cursor_line = 0;
   int conceal_update_lines = FALSE;
 
+  ILOG("Starting Neovim main loop.");
 
   clear_oparg(&oa);
   while (!cmdwin