Fix vtl identifier detection. VTL identifiers must start with either a-zA-Z or an '_' character.
See https://github.com/apache/velocity-engine/blob/trunk/velocity-engine-core/src/main/parser/Parser.jjt#L1135 and http://velocity.apache.org/engine/devel/vtl-reference-guide.html.

Signed-off-by: David Black <dblack@atlassian.com>
Change the existing test_accepts_dollar_digit_identifiers test to test_does_not_accept_dollar_digit_identifiers to indicate that $digit references are not valid.

Signed-off-by: David Black <dblack@atlassian.com>
Merge pull request #9 from dbaxa/fix_vtl_identifiers

Fix vtl identifiers