bibchecker: exclude leading & trailing non-alphabetic characters from expected contents of curly braces

[paxtonfitzpatrick]

When checking entries for words that should be manually capitalized using curly braces (entries in caps.txt), the auto-checker should not expect leading and trailing symbols to be inside the braces. For example, CoppEtal17 was added as:

@inproceedings{CoppEtal17,
	author = {G Coppersmith and C Hilland and O Frieder and R Leary},
	booktitle = {2017 {IEEE} {EMBS} International Conference on Biomedical \& Health Informatics ({BHI})},
	organization = {IEEE},
	pages = {393--396},
	title = {Scalable mental health analysis in the clinical whitespace via natural language processing},
	year = {2017}}

The checker throws the following error:

CoppEtal17: 	booktitle "2017 {IEEE} {EMBS} International Conference on Biomedical \& Health Informatics ({BHI})" should be "2017 {IEEE} {EMBS} International Conference on Biomedical \& Health Informatics {(BHI})"

In the booktitle field, "BHI" in parentheses is capitalized as ({BHI}). The checker sees this as an error and wants {(BHI}) instead.

I'm not familiar with the checker code and don't want to introduce unexpected bugs by trying to change this myself, so for now, I've updated CoppEtal17 to match what the checker prefers.