vcf: separate header lookup from field getter

Create two alternatives to bcf_get_info and bcf_get_fmt allowing the
user to query the header id for the tag once and then accessing the
fields directly using index lookups.  Modified the two existing
functions to use the index lookups so we don't repeat functionality.

Added the following functions to the public API:
```c
bcf_get_fmt_idx (bcf_t* line, const int idx)
bcf_get_info (bcf_t* line, const int idx)
```