added some minor options

[hayguen]

added option -L to print current level values
added option -c to configure de-emphasis time constant for wbfm
added option -v to print verbose (debug) output
added alias modulation names nbfm, nfm, wfm

[hayguen]

• option -L:
  "rtl_fm -L 10 -M fm": prints current level to stderr, every 10th calculation.
  this allows monitoring of the noise (or signal) level, to help setting an optimal squelch threshold.
• option -c:
  "rtl_fm -M wbfm -c 50" uses de-emphasis time constant for europe.
  one might also write "-c us" to use the north american 75 microSec standard.
  "-c eu" will use the european 50 microSec standard.
• option -v:
  "rtl_fm -v 1 -f 99.3M" displays verbose debug information on calculation of frequency, so that user might understand how/what frequency is tuned to.
• modulation names:
  found myself writing "wfm" instead of "wbfm". now the aliases are also accepted.

[hayguen]

added dc filter on raw data (after decimation) with option "-E rdc"
added option "-q" for setting averaging speed for "rdc"
added option "-E adc" in addition to "-E dc"

Please have a look in my mail with subject
"rtl_fm: degraded demodulation caused by self-introduced DC !?"
in osmocom-sdr mailing list to understand the reason for a second dc filter.

[Romeo-Golf]

Seems useful :)