Add a render_flashed_messages macro

[neil-s]

Following up on #4 , and based on bootstrap-flask's implementation, with a name updated to match this project's naming style.

If you are open to this, I'll submit another commit to update the documentation and mention this macro.

I haven't fully tested this with Bootstrap 4.

[coveralls]

Pull Request Test Coverage Report for Build 15

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 88.679%

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Totals
Change from base Build 14:	0.0%
Covered Lines:	47
Relevant Lines:	53

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💛 - Coveralls

[neil-s]

OK this obviously doesn't work yet. Please ignore this PR, I'll update with commits that make it actually work.

[greyli]

Some advice:

• Rewrite it to make it work with Bootstrap 4.
• Let user to pass category name, note that there are 8 category types in Bootstrap 4.
• Match default category (message) to primary alert (class="alert alert-primary").
• Use standard Jinja syntax:
  • Yes: {{ variable }}
  • No: {{variable}}

[SobieskiCodes]

Take it this is no longer on the board? Really the only thing missing.

[greyli]

@SobieskiCodes I'll implement this based on this PR. Please vote for the macro name!

[greyli]

Thanks for working on this! This pr was replaced with #39.