Added directory traversal

[dhruvsomani]

This commit adds the possibility of traversing directories with scspell. Also, the command line argument --ext has been added and can be used more than once and all the files in the directories with those extensions will be checked. In case, the argument is not supplied, all the files in the directory and subdirectories would be checked.

Example: scspell dir1 dir2 --ext py --ext txt This checks for all .py and .txt files in dir1 and dir2.

Closes https://github.com/myint/scspell/issues/22

[dhruvsomani]

@myint Do you know if it's possible for cram to ignore whitespace differences? Because, I work on Windows and it's creating many errors dues to 'LF' and 'CRLF' differences (checked manually, they are all due to mismatching line endings). I converted the input to 'LF' style using dos2unix but am having trouble comparing the output with the tests.

[Edit] This looks like a pending issue.