Add configuration file for heaptrace options

[Bojun-Seo]

The design of option delivering via environmental variable is good if the number of options to deliver is small. But now, it would be better to use configuration file on delivering options, as heaptrace will add ignore option #28 and signal option #33, and something unknown options in the future.
It could be possible to include the contents of ignore.txt in this configuration file for ignore option implementation.

So I suggest configure file format, default name and location. like followings:

• File format: JSON
• Default location: current location
• Default name: .heaptrace.config

Setting configuration file option should be given or delivered via environmental variable. The usage will be:

$ heaptrace --config="/home/name/my_config" a.out

and

$ LD_PRELOAD=/usr/lib/libheaptrace.so HEAPTRACE_CONFIG=/home/name/my_config a.out

Configuration file example:

$ cat .heaptrace.config
{
  "num_top_backtrace": 20,
  "sort_keys": ["size", "count"],
  "out_file": "/tmp/my.log",
  "flame_graph": false,
  "ignore": ["libfoo", "bar", "fac", "baz"],
  "signals": {
    "size": 40,
    "count": 41,
    "slope": 42,
    "r_square": 43
  }
}

I think this change could help users to set options more conveniently. User only need to set configuration file and locate it on the default location or give the location with --config option.