Bisect to add extra link-time flags

[mikebentley15]

Feature Request

Describe the new feature:
Right now, the user specifies compiler flags when calling FLiT Bisect and uses the same link flags as used by the baseline compilation. What if the user requires some more link-time flags, such as an extra rpath or an extra library to be used?

Suggested change:
Add a flag to flit_bisect.py called --ldflags and specify that these flags will be added to the link-time flags usually used for the baseline compilation. I think this is one way to alleviate the question posed in Issue #307.

Also, when --ldflags is used, we would want those flags to replace the link flags specified in flit-config.toml. The alternative is to have them be additive. Replacement gives the user more flexibility. Either way we choose, we would need to document it well.

Perhaps, we can even add another flag to flit_bisect.py called --use-linker where you specify you want a different linker to be used rather than the baseline compiler. I believe we would want this flag to cause us to not use the linker flags specific to the baseline compiler. Additionally, if the given linker program is one of the compilers specified in flit-config.toml, then we would want to use the linker flags specific to that compiler unless --ldflags was specified.

Alternative approaches:
I can't think of any, but I'm open to suggestions.