Add info on archivers/archive formats

docs/archive.md

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+# File Archives & Archivers
+ENiGMA½ can detect and process various archive formats such as zip and arj for a variety of tasks from file upload processing to EchoMail bundle compress/decompression. The `archives` section of `config.hjson` is used to override defaults, add new handlers, and so on.
+
+## Archivers
+Archivers are manged via the `archives:archivers` configuration block of `config.hjson`. Each entry in this section defines an **external archiver** that can be referenced in other sections of `config.hjson` as and in code. Entries define how to `compress`, `decompress` (a full archive), `list`, and `extract` (specific files from an archive).
+
+### Predefined Archivers
+The following archivers are pre-configured in ENiGMA½ as of this writing. Remember that you can override settings or add new handlers!
+
+#### ZZip
+* Formats: .7z, .bzip2, .zip, .gzip/.gz, and more
+* Key: `7Zip`
+* Homepage/package: [7-zip.org](http://www.7-zip.org/). Generally obtained from a `p7zip` package in *nix environments. See http://p7zip.sourceforge.net/ for details.
+
+#### Lha
+* Formats: <a href="https://en.wikipedia.org/wiki/LHA_(file_format)">LHA</a> files such as .lzh.
+* Key: `Lha`
+* Homepage/package: `lhasa` on most *nix environments. See also https://fragglet.github.io/lhasa/
+
+#### Arj
+* Formats: .arj
+* Key: `Arj`
+* Homepage/package: `arj` on most *nix environments.
+
+### Archiver Configuration
+Archiver entries in `config.hjson` are mostly self explanatory with the exception of `list` commands that require some additional information. The `args` member for an entry is an array of arguments to pass to `cmd`. Some variables are available to `args` that will be expanded by the system:
+
+* `{archivePath}` (all): Path to the archive
+* `{fileList}` (compress, extract): List of file(s) to compress or extract
+* `{extractPath}` (decompress, extract): Path to extract *to*
+
+For `list` commands, the `entryMatch` key must be provided. This key should provide a regular expression that matches two sub groups: One for uncompressed file byte sizes (sub group 1) and the other for file names (sub group 2). An optional `entryGroupOrder` can be supplied to change the default sub group order.
+
+#### Example Archiver Configuration
+```
+7Zip: {
+	compress: {
+		cmd: 7za,
+		args: [ "a", "-tzip", "{archivePath}", "{fileList}" ]
+	}
+	decompress: {
+		cmd: 7za,
+		args: [ "e", "-o{extractPath}", "{archivePath}" ]
+	}
+	list: {
+		cmd: 7za,
+		args: [ "l", "{archivePath}" ]
+		entryMatch: "^[0-9]{4}-[0-9]{2}-[0-9]{2}\\s[0-9]{2}:[0-9]{2}:[0-9]{2}\\s[A-Za-z\\.]{5}\\s+([0-9]+)\\s+[0-9]+\\s+([^\\r\\n]+)$",
+	}
+	extract: {
+		cmd: 7za,
+		args [ "e", "-o{extractPath}", "{archivePath}", "{fileList}" ]
+	}
+}
+```
+
+## Archive Formats
+Archive formats can be defined such that ENiGMA½ can detect them by signature or extension, then utilize the correct *archiver* to process them. Formats are defined in the `archives:formats` key in `config.hjson`. Many differnet types come pre-configured (see `core/config.js`).
+
+### Example Archive Format Configuration
+```
+zip: {
+	sig: "504b0304" /* byte signature in HEX */
+	offset: 0
+	exts: [ "zip" ]
+	handler: 7Zip /* points to a defined archiver */
+	desc: "ZIP Archive"
+}
+```