[subset] Added command line options 'text', 'text-file', 'glyphs-file', and 'output'.

- Added command line options (Options attributes):
```
text="" # characters to include in the subset, as UTF-8 string
text_file="" # text file containing characters to include in the subset, as UTF-8 string
glyphs_file="" # file with a list of glyphs, separated by \s
output= "" # name of an output file
```
- Moved 'text' option parsing from main() to Options.parse_opts()
- I used ```re module``` to read the glyphs from glyphs-file path. If that is too slow, I could write something “custom”.
- Another place where I used regexp is removing whitespace characters from text-file input. Perhaps this is unnecessary?
- Remove the glyphs-file option if it seems unnecessary. It seemed useful to me as that is how I prepare the subset definitions anyway.