Add option to `fitsheader` to extract a specific keyword

This PR implements the feature requested in #2555. That is, I have added an optional `--keyword` argument to the `fitsheader` script to allow one or more specific keyword value(s) to be retrieved from the header of a FITS file.

The proposed behaviour is described below. Please review.

Updated usage info:
```
$ fitsheader -h
usage: fitsheader [-h] [-e HDU] [-k KEYWORD] [-c] filename [filename ...]

Print the header(s) of a FITS file. All HDU extensions are shown by default.
In the case of a compressed image, the decompressed header is shown.

positional arguments:
  filename              path to one or more FITS files

optional arguments:
  -h, --help            show this help message and exit
  -e HDU, --ext HDU     specify the HDU extension by name or number
  -k KEYWORD, --keyword KEYWORD
                        retrieve the value for a specific keyword, or for a
                        comma-delimited list of keywords
  -c, --compressed      for compressed image data, show the true header which
                        describes the compression rather than the data
```


Example 1: extracting the value of the BITPIX keyword in HDU #0:
```
$ fitsheader --ext 0 --keyword BITPIX tests/data/test0.fits
16
```

Example 2: multiple keywords can be requested using a comma-delimited list. Values are printed in the requested order, one value per line:
```
$ fitsheader --ext 0 --keyword BITPIX,DATE-OBS,TIME-OBS tests/data/test0.fits
16
19/05/94
15:41:16
```

Example 3: when no single HDU is specified, the keyword is shown for each HDU. Again, one value per line:
```
$ fitsheader --keyword BITPIX tests/data/test0.fits 
16
16
16
16
16
```

Example 4: when a keyword does not exist, a warning message is shown and a blank line is returned to stdout:
```
$ fitsheader --ext 2 --keyword BLABLA tests/data/test0.fits
WARNING: tests/data/test0.fits (HDU 2): Keyword BLABLA not found. [astropy.io.fits.scripts.fitsheader]

```

Is this what you had in mind, @bsipocz?
The only thing I think I would want to change here is to change this to `--keyword`.  More verbose, yes, but also keeping in consistency with the API which has standardized on using the term "keyword", which is clearer and more accurate when it comes to FITS files specifically.