Including templates fails on Windows

[smainz]

Parsing of the migrations in ./sample fails on Windows.

This is from current master branch:

PS C:\Users\user\Projects\tern> .\tern gengen --migrations sample
Error loading migrations:
  template: 005_create_v1.sql up:1:12: executing "005_create_v1.sql up" at <{{template "shared/v1_001.sql" .}}>: template "shared/v1_001.sql" not defined
PS C:\Users\user\Projects\tern>

PR for the fix is on it's way.