`Base.parse` and `Base.tryparse` for `DNA` and `RNA`

[warrenalphonso]

Add Base.parse and Base.tryparse for DNA and RNA

Fix #50. I think this behaviour makes sense for a Base.parse implementation for these two types. I'm really new to Julia so feel free to let me know if there's anything I can improve :)

Types of changes

This PR implements the following changes: (Please tick any or all of the following that are applicable)

• [x] :sparkles: New feature (A non-breaking change which adds functionality).
• [ ] :bug: Bug fix (A non-breaking change, which fixes an issue).
• [ ] :boom: Breaking change (fix or feature that would cause existing functionality to change).

:ballot_box_with_check: Checklist

• [x] :art: The changes implemented is consistent with the julia style guide.
• [ ] :blue_book: I have updated and added relevant docstrings, in a manner consistent with the documentation styleguide.
• [ ] :blue_book: I have added or updated relevant user and developer manuals/documentation in docs/src/.
• [x] :ok: There are unit tests that cover the code changes I have made.
• [x] :ok: The unit tests cover my code changes AND they pass.
• [x] :pencil: I have added an entry to the [UNRELEASED] section of the manually curated CHANGELOG.md file for this repository.
• [x] :ok: All changes should be compatible with the latest stable version of Julia.
• [x] :thought_balloon: I have commented liberally for any complex pieces of internal code.