Сalculate sourceMappingURL size

[nikolay-borzov]

• Add API excludeSourceMapComment option and exclude-source-map CLI parameter to exclude sourceMappingURL comment from size analysis
• Rename <unmapped> to [unmapped]. Using <> in HTML output cause wrong title <
• Export UNMAPPED_KEY SOURCE_MAP_COMMENT_KEY from index.ts so that these virtual files can be found among files object properties
• Improve size analysis accuracy. Calculate sourceMappingURL comment and EOL bytes. Use Buffer.byteLength to get string length in bytes. UTF-8 encoding is assumed. We can add encoding detection later if needed, but it isn't an easy thing to do.
• Add some tests

Fixes #133

[coveralls]

Coverage increased (+1.2%) to 98.769% when pulling 28953c0e5ef8a40823910da74691c66485ecbd8b on calculate-sourceMappingURL-size into 98b13b2fb69944e8a5aa61c75cf5a47a60f55fcc on master.