add @primary_extension_index to ensure we don't have duped primary exts
use @primary_extension_index in find_by_filename
change MUMPS' primary extension from .m to .mumps

.m currently clashes with Objective-C, and MUMPS is chosen before
Objective-C when creating .m files in Gist. Even if .mumps isn't a
legitimate MUMPS extension, I think we should change it anyway since
Objective-C is far more likely to be intended when a user uses .m
add .m as an alternate extension for MUMPS so it's still picked up
update samples.json
Merge pull request #728 from github/fix-dup-primary-extension-for-mumps

Ensure no two languages have the same primary extension