Runes in ctlk core

[clemsciences]

This is my pull request following #794. The runes retrieval method will be added in a specific repository.

[codecov-io]

Codecov Report

Merging #796 into master will increase coverage by 0.02%.
The diff coverage is 93.33%.

@@            Coverage Diff             @@
##           master     #796      +/-   ##
==========================================
+ Coverage   88.49%   88.52%   +0.02%     
==========================================
  Files         161      162       +1     
  Lines       10155    10215      +60     
==========================================
+ Hits         8987     9043      +56     
- Misses       1168     1172       +4

Impacted Files	Coverage Δ
cltk/tests/test_corpus.py	99.21% <100%> (+0.01%)	⬆️
cltk/corpus/old_norse/runes.py	91.83% <91.83%> (ø)

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[Sedictious]

This is a great start! It may need some refactoring later on, should you want to add more features, but as of now, this should suffice!

[clemsciences]

I need this step to be completed because I want to:

• add more runic inscriptions,
• use it for a Jupyter notebook in the tutorials,
• to develop more features, it is easier to have the basic code already in the core code.

If you @Sedictious want to write code for runes, you can do it! Moreover, all the Germanic runes may be in a unique module, so some refactor will be needed