Fix problems with rendering and formatting of combined unit field

[PhilippImhof]

This PR fixes the problems in #267 and #266. It has two changes:

• Prior to Implement custom formatting of input fields #235, users could specify {_0}{_u} (with no space) in order to obtain a combined unit field. As there can now be options, it's possible to have {_0|width=100px}{_u|bgcol=red}. Unfortunately, I forgot that and was still searching for just {_0}{_u}.
• Also, insertion of the combined field, once it was rendered, occurred at the position of the {_0}{_u} placeholders. I forgot to dynamically create the search string, taking into account the actual placeholder with its formatting options.

If the user adds formatting to both the {_0} and the {_u} part, the plugin will merge the options. In case of conflicts, the options from the {_u} field (the latter) will be used.

[coveralls]

Pull Request Test Coverage Report for Build 16798398940

Details

• 11 of 11 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.004%) to 97.633%

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Totals
Change from base Build 16770520117:	0.004%
Covered Lines:	4331
Relevant Lines:	4436

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💛 - Coveralls

[PhilippImhof]

@dbauer-ets If you have some time, please have a look at the fix. It works fine in my tests, I have also added two specific unit tests to cover the two bugs. I'd love to merge it by the end of the week and would then publish a bugfix release.