Fix issue where CustomSeverities are not respected in config

Original ability was added in: #281

When trying out the functionality, it does not respect custom_severities
as documented in the README.

This patch addresses it by including the porting of custom severities
from the the same location where other config keys are copied

I've confirmed that it works both in a new test and in manual regression
testing.

Fixes: #364

talismanrc/talismanrc.go

@@ -201,6 +201,7 @@ func fromPersistedRC(configFromTalismanRCFile *persistedRC, mode Mode) *Talisman
 	tRC.ScopeConfig = configFromTalismanRCFile.ScopeConfig
 	tRC.Experimental = configFromTalismanRCFile.Experimental
 	tRC.CustomPatterns = configFromTalismanRCFile.CustomPatterns
+	tRC.CustomSeverities = configFromTalismanRCFile.CustomSeverities
 	tRC.Experimental = configFromTalismanRCFile.Experimental
 	tRC.AllowedPatterns = make([]*regexp.Regexp, len(configFromTalismanRCFile.AllowedPatterns))
 	for i, p := range configFromTalismanRCFile.AllowedPatterns {

talismanrc/talismanrc_test.go

@@ -78,6 +78,19 @@ func TestShouldConvertThresholdToValue(t *testing.T) {
 	assert.Equal(t, newPersistedRC(talismanRCContents).Threshold, severity.High)
 }
 
+func TestObeysCustomSeverityLevelsAndThreshold(t *testing.T) {
+	talismanRCContents := []byte(`threshold: high
+custom_severities:
+- detector: Base64Content
+  severity: low
+`)
+	persistedRC := newPersistedRC(talismanRCContents)
+	talismanRC := fromPersistedRC(persistedRC, ScanMode)
+	assert.Equal(t, newPersistedRC(talismanRCContents).Threshold, severity.High)
+	assert.Equal(t, len(newPersistedRC(talismanRCContents).CustomSeverities), 1)
+	assert.Equal(t, persistedRC.CustomSeverities, talismanRC.CustomSeverities)
+}
+
 func TestDirectoryPatterns(t *testing.T) {
 	assertAccepts("foo/", "", "bar", t)
 	assertAccepts("foo/", "", "foo", t)