MNT: New custominstall make rule

Paul McCarthyrequested to merge
mnt/update_fsl_package into master

Can optionally be defined in project Makefile for any custom installation steps, e.g. the hacky schedule rule used by flirt and feat5. The custominstall rule is executed when make install is run, unlike the afore-mentioned schedule rules, which are added to the all: target, and hence have been getting executed when make is run.

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