Generate manpage as part of build

[lafrenierejm]

Fixes #73 by generated a manpage via clap_mangen.

This is the output nroff I get when building locally.

.ie \n(.g .ds Aq \(aq
.el .ds Aq '
.TH ripsecrets 1  "ripsecrets 0.1.7" 
.SH NAME
ripsecrets \- A command\-line tool to prevent committing secret keys into your source code
.SH SYNOPSIS
\fBripsecrets\fR [\fB\-\-install\-pre\-commit\fR] [\fB\-\-strict\-ignore\fR] [\fB\-\-only\-matching\fR] [\fB\-\-additional\-pattern\fR] [\fB\-h\fR|\fB\-\-help\fR] [\fB\-V\fR|\fB\-\-version\fR] [\fISource files\fR] 
.SH DESCRIPTION
ripsecrets searches files and directories recursively for secret API keys.
It\*(Aqs primarily designed to be used as a pre\-commit to prevent committing
secrets into version control.
.SH OPTIONS
.TP
\fB\-\-install\-pre\-commit\fR
Install `ripsecrets` as a pre\-commit hook automatically in git directory provided
.TP
\fB\-\-strict\-ignore\fR
If you pass a path as an argument that\*(Aqs ignored by .secretsignore it will be scanned by default. \-\-strict\-ignore will override this behavior and not search the paths passed as arguments that are excluded by the .secretsignore file. This is useful when invoking secrets as a pre\-commit
.TP
\fB\-\-only\-matching\fR
Print only the matched (non\-empty) parts of a matching line, with each such part on a separate output line
.TP
\fB\-\-additional\-pattern\fR=\fIADDITIONAL_PATTERNS\fR
Additional regex patterns used to find secrets. If there is a matching group in the regex the matched group will be tested for randomness before being reported as a secret
.TP
\fB\-h\fR, \fB\-\-help\fR
Print help (see a summary with \*(Aq\-h\*(Aq)
.TP
\fB\-V\fR, \fB\-\-version\fR
Print version
.TP
[\fISource files\fR]
Source files. Can be files or directories. Defaults to \*(Aq.\*(Aq
.SH VERSION
v0.1.7

[lafrenierejm]

@sirwart Any opinions on this PR?

[sirwart]

It looks good, thanks!