Added the option to read from a previously saved file of socket30003 output.

[icthieves]

the -readcsv option takes a file in dump1090 socket30003 format and processes it.
This is just a hacky wrapper around the existing parsing loop.
If the $infile variable is defined, $SOCKET represents the file, instead of a socket directly.

example:
./socket30003.pl -readcsv="/path/to/captured/30003.csv"

I tried to write everything in the same style as the original work, though the last line of logging (line 796) may be using LOG() arguments you want to change (currently, "W", when the file is finished processing)

Other considerations:

• The $infile variable should only be checked once for each socket connection or reconnection, So i don't expect any performance hit.
• the readcsv option is documented and handled in the same way as all other CLI options
• i've tested it with a ~30MB file i was capturing today (not being able to make a heatmap with it is what generated this pull request)
• Here's a screenshot of the heatmap generated from one day's worth of captured socket data.
  I have no idea if this is typically for such a sparse heatmap, as i had a very small data set. It is included here for your judgement.
  
• The data was captured with

sudo wget -O - -q http://localhost:30003 >> /path/to/flights.csv

I'm planning to collect another few days worth of data in CSV format to test the change further, but it seems to be working to the best of my knowledge.

Thanks for your work on dump1090, I just got my rtl-sdr yesterday and i'm having a blast!

Sidenote: It seems the heatmap is offset a few degrees horizontally from the Airport (YVR, on Sea Island, just above Richmond in the picture).
Clearly the google map is slightly off, but i'm not sure how to correct it.