memory and tag exclusion

• Lower memory usage when processing intervals and computing coverage statistics, by reducing interval storage overhead and avoiding coverage-array copies. Thanks @DylanCosto! #273
• New --exclude-tag TAG:VALUE option to filter reads by exact string-tag values, with support for comma-separated filters. For example, -F 772 --exclude-tag DT:SQ excludes optical duplicates while keeping PCR/library duplicates. Thanks @bwlang! #272
• Fixed a documentation typo. Thanks @pikammmmm! #267