Add underline support

[Bartemis]

Would be nice to add underline support.
To do it now you can use the following code:

import jira2markdown
from jira2markdown.elements import MarkupElements
from jira2markdown.markup.text_effects import QuotedElement, Underline

text = "This is a +test+"


class CustomUnderline(QuotedElement):
    TOKEN = "+"
    QUOTE_CHAR = "<u>"
    END_QUOTE_CHAR = "</u>"


elements = MarkupElements()
elements.replace(Underline, CustomUnderline)

print(jira2markdown.convert(text, elements=elements))

[catcombo]

Hi @Bartemis, thank you for you proposal. Could you please test updated code with your test cases before I publish it? See #15

[Bartemis]

Hi @catcombo, I test this with success

[catcombo]

@Bartemis great, thank you! I'll merge the PR and will publish a new version.