Add { relative: ... } support for replacements configuration option

[xkr47]

so I can do:

"replacements": {
"res/(.*)": {
"relative": "./$1"
}
}

untested.

[coveralls]

Coverage decreased (-1.1%) to 93.827% when pulling 8ac1c7b on xkr47:patch-1 into 6dba2aa on benbria:master.

[coveralls]

Coverage increased (+0.1%) to 95.062% when pulling 7952b86 on xkr47:patch-1 into 6dba2aa on benbria:master.